The idea of making it easier to interact with low level hardware from a PC is not new. Several solutions to this problem have been developed, each with its own advantages and drawbacks. Some examples will be presented in this chapter.
The idea of making it easier to interact with low level hardware from a PC is not new. Several solutions to this problem have been developed, each with its own advantages and drawbacks. Some examples will be presented in this chapter.
\caption{\label{fig:rpi}Raspberry Pi minicomputers}
\end{figure}
\end{figure}
\todo[inline]{link to pic source page}
The Raspberry Pi's GPIO header, a row of pins which can be directly controlled by user applications running on the minicomputer, was one of the inspirations behind GEX. It can be controlled using C and Python (among others) and offers general purpose I/O, SPI, I2C, UART and PWM, with other protocols easy to emulate thanks to the high speed of the system processor.
%http://dangerousprototypes.com/blog/about/
The Raspberry Pi is used in schools as a low-cost PC alternative that encourage students' interest in STEM (Science, Technology, Engineering and Mathematics). The board is often built into more permanent projects that make use of its powerful processor, such as wildlife camera traps, fish feeders etc.
Bus Pirate, developed by \todo{link}Ian Lesnet at Dangerous Prototypes and manufactured by Seeed Studio\todo{link}, is a USB-attached device providing access to hardware interfaces like SPI, I$^2$C, USART and 1-Wire, as well as frequency measurement and direct pin access.
The board aims to make it easy for users to familiarize themselves with new chips and modules; it also provides a range of programming interfaces for flashing microcontroller firmwares and memories. It communicates with the PC using a FTDI USB-serial bridge.
Bus Pirate is open source and in scope it's similar to GEX. It can be scripted and controlled from languages like Python or Perl, connects to USB and provides a good selection of hardware interfaces.
The Raspberry Pi could be used for the same quick evaluations or experiments we want to perform with GEX, however they would either have to be performed directly on the mini-computer, with an attached monitor and a keyboard, or use some form of remote access (e.g. SSH, screen sharing).
The board is based on a PIC16 microcontroller running at 32\,MHz. Its analog/digital converter (ADC) only has a resolution of 10 bits (1024 levels). There is no digital/analog converter (DAC) available on the chip, making applications that require a varied output voltage more difficult. Another limitation of the board is its low number of GPIO pins which may be insufficient for certain applications. The Bus Pirate, at the time of writing, can be purchased for a price similar to some Raspberry Pi models.
\caption{\label{fig:rpi2}Raspberry Pi 2 (picture by \textit{Raspberry Pi Foundation})}
\caption{\label{fig:buspirate}Bus Pirate v.4 (picture by \textit{Seeed Studio})}
\end{figure}
\end{figure}
The Raspberry Pi's GPIO header, which can be directly controlled by user applications, was one of the primary inspirations behind GEX. It can be controlled using C and Python (among others) and offers general purpose I\O, SPI, I2C, UART and PWM, with other protocols being easy to emulate thanks to the high speed of the system processor.
%http://dangerousprototypes.com/blog/about/
% Dangerous Prototypes and manufactured by Seeed Studio\todo{link}
Bus Pirate, a project by Ian Lesnet, is a USB-attached device providing access to hardware interfaces like SPI, I$^2$C, USART and 1-Wire, as well as frequency measurement and direct pin access. The board aims to make it easy for users to familiarize themselves with new chips and modules; it also provides a range of programming interfaces for flashing microcontroller firmwares and memories. It communicates with the PC using a FTDI USB-serial bridge.
The Raspberry Pi is commonly used in schools as a low-cost PC alternative that encourage students' interest in electronics, programming and science. The board is often built into more permanent projects that make use of its powerful processor, such as wildlife camera traps or home automation projects.
Bus Pirate is open source and is, in its scope, similar to GEX. It can be scripted and controlled from the PC, connects to USB and provides a wide selection of hardware interfaces.
The Raspberry Pi could be used for the same quick evaluations or experiments we want to perform with GEX, however they would either have to be performed directly on the mini-computer itself with attached monitor and keyboard, or use some form of remote access (e.g. SSH). When we have a more powerful computer available, a USB device like GEX would be more convenient.
The board is based on a PIC16 microcontroller running at 32\,MHz. Its analog/digital converter (ADC) only has a resolution of 10 bits (1024 levels). There is no digital/analog converter (DAC) available on the chip, making applications that require a varied output voltage more difficult. Another limitation of the board is its low number of GPIO pins, which may be insufficient for certain applications. The Bus Pirate is available for purchase at around 30\,USD, a price comparable to some Raspberry Pi models.
\section{Professional DAQ Modules}
\section{Professional DAQ Modules}
Various professional tools that would fulfill our needs exist on the market, but their high price makes them inaccessible for users with a limited budget, such as hobbyists or students who would like to keep such a device for personal use. An example is the National Instruments (NI) "I²C/SPI Interface Device" which also includes several GPIO lines, the NI USB DAQ module, or some of the Total Phase I²C/SPI gadgets (figure \ref{fig:profidaq}). The performance GEX can provide may not always match that of those professional tools, but in many cases it'll be a sufficient substitute at a fraction of the cost.
Various professional tools that would fulfill our needs exist on the market, but their high price makes them inaccessible for users with a limited budget, such as hobbyists or students who would like to keep such a device for personal use. An example is the National Instruments (NI) "I²C/SPI Interface Device" which also includes several GPIO lines, the NI USB DAQ module, or some of the Total Phase I²C/SPI gadgets (figure \ref{fig:profidaq}). The performance GEX can provide may not always match that of those professional tools, but in many cases it'll be a sufficient substitute at a fraction of the cost.
\caption[Professional tools that GEX can replace]{\label{fig:profidaq}An example of professional tools that GEX could replace in less demanding scenarios (pictures taken from marketing materials)}
\caption[Professional tools that GEX can replace]{\label{fig:profidaq}An example of professional tools that GEX could replace in less demanding scenarios \textit{(pictures from National Instruments and Total Phase marketing materials)}}
@ -69,4 +76,5 @@ Various professional tools that would fulfill our needs exist on the market, but
Firmata is a communication protocol based on MIDI (\textit{Musical Instrument Digital Interface}) for passing data to and from embedded microcontrollers. MIDI is mainly used for attaching electronic musical instruments, such as synthesizers, keyboards, mixers etc., to each other or to a PC. Firmata was designed for Arduino as a high level abstraction for its connection to the PC, typically using a FTDI chip or equivalent.
Firmata is a communication protocol based on MIDI (\textit{Musical Instrument Digital Interface}) for passing data to and from embedded microcontrollers. MIDI is mainly used for attaching electronic musical instruments, such as synthesizers, keyboards, mixers etc., to each other or to a PC. Firmata was designed for Arduino as a high level abstraction for its connection to the PC, typically using a FTDI chip or equivalent.
Implementing Firmata in the GEX firmware would make it possible to use existing Firmata libraries on the PC side. However, the protocol is limited by the encompassing MIDI format and isn't very flexible.
Implementing Firmata in the GEX firmware would make it possible to use existing Firmata libraries on the PC side. However, the protocol is limited by the encompassing MIDI format and isn't very flexible.
@ -67,8 +67,6 @@ As the settings persist after a firmware update, it's important to maintain back
The INI files, which can be edited through the communication API or using a text editor with the virtual mass storage, are parsed and generated on demand and are never stored in the Flash or RAM, other than in short temporary buffers. The INI parser processes the byte stream on-the-fly as it is received, and a similar method is used to build a INI file from the configured units and system settings.
The INI files, which can be edited through the communication API or using a text editor with the virtual mass storage, are parsed and generated on demand and are never stored in the Flash or RAM, other than in short temporary buffers. The INI parser processes the byte stream on-the-fly as it is received, and a similar method is used to build a INI file from the configured units and system settings.
\todo[inline]{add a sample large INI file as an attachment}
GEX's user-facing functions, also called functional blocks or \textit{units}, are implemented in \textit{unit drivers}. Those are independent modules in the firmware that the user can enable and configure using the GEX configuration files. In principle, there can be multiple instances of each unit type. However, we are limited by hardware constraints: there may be only one ADC peripheral, two SPI ports and so on. The mutually exclusive assignment of resources to units is handled by the \textit{resource registry} (\ref{sec:res-allocation}).
GEX's user-facing functions, also called functional blocks or \textit{units}, are implemented in \textit{unit drivers}. Those are independent modules in the firmware that the user can enable and configure using the GEX configuration files. In principle, there can be multiple instances of each unit type. However, we are limited by hardware constraints: there may be only one ADC peripheral, two SPI ports and so on. The mutually exclusive assignment of resources to units is handled by the \textit{resource registry} (\ref{sec:res-allocation}).
@ -147,7 +145,7 @@ This is identical to the USB connection from the PC application's side, except a
The wireless connection uses an on-board communication module and a separate device, a wireless gateway, that connects to the PC. The wireless gateway is interfaced differently from the GEX board itself, but it also shows as a virtual serial port on the host PC. This is required to allow communicating with the gateway itself through the CDC/ACM interface in addition to addressing the end devices.
The wireless connection uses an on-board communication module and a separate device, a wireless gateway, that connects to the PC. The wireless gateway is interfaced differently from the GEX board itself, but it also shows as a virtual serial port on the host PC. This is required to allow communicating with the gateway itself through the CDC/ACM interface in addition to addressing the end devices.
This interface will be explained in more detail in chapter XX \todo{Link to tinyframe description}.
This interface will be explained in more detail in chapter \ref{sec:wireless}.
@ -6,7 +6,7 @@ Hardware buses implemented in GEX are presented in this chapter. The description
The \textit{Universal Synchronous / Asynchronous Receiver Transmitter} has a long history and is still in widespread use today. It is the protocol used in RS-232, which was once a common way of connecting modems, printers, mice and other devices to personal computers. UART framing is also used in the industrial bus RS-485.
The \textit{Universal Synchronous / Asynchronous Receiver Transmitter} has a long history and is still in widespread use today. It is the protocol used in RS-232, which was once a common way of connecting modems, printers, mice and other devices to personal computers. UART framing is also used in the industrial bus RS-485.
\caption[UART frame structure]{\label{fig:uart-frame}UART frame, as shown by the STM32F072 Reference Manual. Break frames are used by some UART based protocols, like LIN (Local Interconnect Network).}
\caption[UART frame structure]{\label{fig:uart-frame}UART frame, as shown by the STM32F072 Reference Manual. Break frames are used by some UART based protocols, like LIN (Local Interconnect Network).}
@ -35,12 +35,13 @@ Transmission and reception on the SPI bus happen simultaneously. A bus master as
SPI devices often provide a number of control, configuration and status registers that can be read and written by the bus master. The first byte of a command usually contains one bit that determines if it's a read or write access, and an address field selecting the target register.
SPI devices often provide a number of control, configuration and status registers that can be read and written by the bus master. The first byte of a command usually contains one bit that determines if it's a read or write access, and an address field selecting the target register.
\caption[SPI master with multiple slaves]{\label{fig:spi-multislave}A SPI bus with 1 master and 3 slaves, each enabled by its own Slave Select signal (\textit{STM32F072 Reference Manual})}
\caption[SPI master with multiple slaves]{\label{fig:spi-multislave}A SPI bus with 1 master and 3 slaves, each enabled by its own Slave Select signal (\textit{STM32F072 Reference Manual})}
\end{figure}
\end{figure}
\pagebreak[1]% TODO
\subsection{Examples of Devices Using SPI}
\subsection{Examples of Devices Using SPI}
\begin{itemize}
\begin{itemize}
@ -63,7 +64,7 @@ I2C is a two-wire (SDA--\textit{Serial Data}, SCL--\textit{Serial Clock}), open-
I2C uses two addressing modes: 7-bit and 10-bit. Due to the small address space, exacerbated by many devices implementing only the 7-bit addressing, collisions between chips from different manufacturers are common; many devices thus offer several pins to let the board designer choose a few bits of the address by connecting them to different logic levels. I2C allows slow slave devices to stop the master from sending more data by holding the SCL line low at the end of a byte. As the bus is open-drain, the line can't go high until all participants release it. This function is called \textit{Clock Stretching}.
I2C uses two addressing modes: 7-bit and 10-bit. Due to the small address space, exacerbated by many devices implementing only the 7-bit addressing, collisions between chips from different manufacturers are common; many devices thus offer several pins to let the board designer choose a few bits of the address by connecting them to different logic levels. I2C allows slow slave devices to stop the master from sending more data by holding the SCL line low at the end of a byte. As the bus is open-drain, the line can't go high until all participants release it. This function is called \textit{Clock Stretching}.
\caption[I2C message diagram]{\label{fig:i2c-frame}An I2C message diagram. The frame starts with a start condition and stops with a stop condition, defined by an SDA edge while SCL is high. The address and data bytes are acknowledged by the slave by sending a 0 on the open-drain SDA line in the following clock cycle. A slave can terminate the transaction by sending 1 in place of the acknowledge bit. (\textit{Diagram taken from the I2C specification UM10204 by NXP Semiconductors})}
\caption[I2C message diagram]{\label{fig:i2c-frame}An I2C message diagram. The frame starts with a start condition and stops with a stop condition, defined by an SDA edge while SCL is high. The address and data bytes are acknowledged by the slave by sending a 0 on the open-drain SDA line in the following clock cycle. A slave can terminate the transaction by sending 1 in place of the acknowledge bit. (\textit{Diagram taken from the I2C specification UM10204 by NXP Semiconductors})}
@ -87,7 +88,7 @@ The 1-Wire bus, developed by Dallas Semiconductor, uses a single bi-directional
1-Wire is open-drain and the communication consists of short pulses sent by the master and (for bit reading) the line continuing to be held low by the slave. The pulse timing (fig. \ref{fig:1w-pulses}) defines if it's a read or write operation and what bit value it carries. A transaction is started by a 480us long "reset" pulse send by master and ended by a 1-byte CRC checksum.
1-Wire is open-drain and the communication consists of short pulses sent by the master and (for bit reading) the line continuing to be held low by the slave. The pulse timing (fig. \ref{fig:1w-pulses}) defines if it's a read or write operation and what bit value it carries. A transaction is started by a 480us long "reset" pulse send by master and ended by a 1-byte CRC checksum.
@ -104,7 +105,9 @@ The 1-Wire bus, developed by Dallas Semiconductor, uses a single bi-directional
Since 1-Wire is a proprietary protocol, there is a much smaller choice of available devices and they also tend to be more expensive. The DS18x20 thermometers are, however, popular enough to warrant the bus's inclusion in GEX.
Since 1-Wire is a proprietary protocol, there is a much smaller choice of available devices and they also tend to be more expensive. The DS18x20 thermometers are, however, popular enough to warrant the bus's inclusion in GEX.
@ -115,19 +118,35 @@ Since 1-Wire is a proprietary protocol, there is a much smaller choice of availa
NeoPixel is a marketing name of the \textbf{WS2811}, \textbf{WS2812} and compatible intelligent LED drivers that is commonly used in "addressable LED strips". Those chips include the control logic, PWM drivers and usually the LED diodes all in one miniature package.
NeoPixel is a marketing name of the \textbf{WS2811}, \textbf{WS2812} and compatible intelligent LED drivers that is commonly used in "addressable LED strips". Those chips include the control logic, PWM drivers and usually the LED diodes all in one miniature package.
The NeoPixel protocol is unidirectional, using only one data pin. The LED drivers are chained together. Ones and zeros are encoded by a pulse length on the data pin; after loading the color data to the LED string, a longer "reset" pulse is issued by the bus master and the set colors are displayed. The timing diagram and constraints are shown in figure \ref{fig:ws2812-dia}.
The NeoPixel protocol is unidirectional, using only one data pin. The LED drivers are chained together. Ones and zeros are encoded by a pulse length on the data pin; after loading the color data to the LED string, a longer "reset" pulse is issued by the bus master and the set colors are displayed. The timing diagram and constraints are shown in table \ref{fig:ws2812-dia}.
The NeoPixel timing is very sensitive to pulse length accuracy. Reliable ways to implement it use DMA with a hardware timer, or a I2S peripheral. An easier method that does not use any additional hardware resources is implementing the protocol as delay loops in the firmware; care must be taken to disable interrupts in the sensitive parts of the timing.
The NeoPixel timing is very sensitive to pulse length accuracy. Reliable ways to implement it use DMA with a hardware timer, or a I2S peripheral. An easier method that does not use any additional hardware resources is implementing the protocol as delay loops in the firmware; care must be taken to disable interrupts in the sensitive parts of the timing.
In addition to communication buses, described in chapter \ref{ch:hw_buses}, GEX implements several measurement and output functions that take advantage of the microcontroller's peripheral blocks, such as timers/counters and DAC. The more complicated ones are described here; simpler functions, such as the raw GPIO access, will be described later together with their control API.
In addition to communication buses, described in chapter \ref{ch:hw_buses}, GEX implements several measurement and output functions that take advantage of the microcontroller's peripheral blocks, such as timers/counters and DAC. The more complicated ones are described here; simpler functions, such as the raw GPIO access, will be described later together with their control API.
@ -6,24 +6,6 @@ In addition to communication buses, described in chapter \ref{ch:hw_buses}, GEX
Applications like motor speed measurement and the reading of a VCO (voltage-controlled-oscillator) or VCO-based sensor's output demand a tool capable of measuring frequency. This can be done using a laboratory instrument such as the Agilent 53131A. A low cost solution is to use a timer/counter peripheral of a microcontroller, such as the STM32F072 used in GEX.
Applications like motor speed measurement and the reading of a VCO (voltage-controlled-oscillator) or VCO-based sensor's output demand a tool capable of measuring frequency. This can be done using a laboratory instrument such as the Agilent 53131A. A low cost solution is to use a timer/counter peripheral of a microcontroller, such as the STM32F072 used in GEX.
\begin{figure}
\centering
\includegraphics[scale=1]{img/fcap-direct.pdf}
\caption{\label{fig:fcap-direct-dia}Direct frequency measurement method}
\caption[Frequency measurement methods comparison]{\label{fig:freqmethods-graph}Worst-case error using the two frequency measurement methods with an ideal 48\,MHz timer clock. The crossing lies at 7\,kHz with an error of 0.015\,\%, or 1.05\,Hz.}
\end{figure}
Two basic methods to measure frequency exist, each with it's advantages and drawbacks:
Two basic methods to measure frequency exist, each with it's advantages and drawbacks:
\begin{itemize}
\begin{itemize}
@ -40,19 +22,58 @@ Two basic methods to measure frequency exist, each with it's advantages and draw
Te reciprocal method's resolution depends on the counter's clock speed; if driven at 48\,MHz, the tick period is 20.83\,ns, which defines the granularity of our time measurement. It is common to measure several pulses and average the obtained values to further increase the precision.
Te reciprocal method's resolution depends on the counter's clock speed; if driven at 48\,MHz, the tick period is 20.83\,ns, which defines the granularity of our time measurement. It is common to measure several pulses and average the obtained values to further increase the precision.
We can easily achieve a sub-hertz resolution with this method, but its performance degrades at high frequencies where the time measurement precision becomes insufficient. The input frequency range can be extended using a hardware prescaller\footnote{\textit{Prescaller} is a divider implemented as part of the timer/counter peripheral block that can be optionally enabled and configured to a desired division factor.}, which is also applicable to the direct method, should the measurement of frequencies outside the counter's supported range be required. A duty cycle measurement available in this method can be used to read the output of sensors that use a pulse-width modulation.
We can easily achieve a sub-hertz resolution with this method, but its performance degrades at high frequencies where the time measurement precision becomes insufficient. The input frequency range can be extended using a hardware prescaller\footnote{\textit{Prescaller} is a divider implemented as part of the timer/counter peripheral block that can be optionally enabled and configured to a desired division factor.}, which is also applicable to the direct method, should the measurement of frequencies outside the counter's supported range be required. A duty cycle measurement available in this method can be used to read the output of sensors that use a pulse-width modulation.
\end{itemize}
\end{itemize}
\begin{figure}[h]
\centering
\includegraphics[scale=1]{img/fcap-direct.pdf}
\caption{\label{fig:fcap-direct-dia}Direct frequency measurement method}
\caption{\label{fig:fcap-reci-dia}Reciprocal frequency measurement method}
\end{figure}
Which method to use depends on the frequency we want to measure; the worst-case measurement errors of both methods, assuming an ideal 48\,MHz system clock, are plotted in figure \ref{fig:freqmethods-graph}. It can be seen that the reciprocal method leads in performance up to 7\,kHz where the direct method overtakes it. If a higher error is acceptable, the reciprocal method could be used also for higher frequencies to avoid a reconfiguration and to take advantage of its higher speed.
Which method to use depends on the frequency we want to measure; the worst-case measurement errors of both methods, assuming an ideal 48\,MHz system clock, are plotted in figure \ref{fig:freqmethods-graph}. It can be seen that the reciprocal method leads in performance up to 7\,kHz where the direct method overtakes it. If a higher error is acceptable, the reciprocal method could be used also for higher frequencies to avoid a reconfiguration and to take advantage of its higher speed.
\caption[Frequency measurement methods comparison]{\label{fig:freqmethods-graph}Worst-case error using the two frequency measurement methods with an ideal 48\,MHz timer clock. The crossing lies at 7\,kHz with an error of 0.015\,\%, or 1.05\,Hz.}
\end{figure}
A good approach to a universal measurement, when we don't know the expected frequency beforehand, could be to first obtain an estimate using the direct method, and if the frequency is below the worst-case error crossing point (here 7\,kHz), to take a more precise measurement using the reciprocal method.
A good approach to a universal measurement, when we don't know the expected frequency beforehand, could be to first obtain an estimate using the direct method, and if the frequency is below the worst-case error crossing point (here 7\,kHz), to take a more precise measurement using the reciprocal method.
The system clock's frequency, which we use to measure pulse lengths and to gate the pulse counter, will be affected by tolerances of the used components, the layout of the PCB, temperature effects etc., causing measurement errors. A higher accuracy could be achieved using a temperature-compensated oscillator (TCO), or, in the direct method, by using the synchronization pulse provided by a GPS receiver to time the measurement interval.
The system clock's frequency, which we use to measure pulse lengths and to gate the pulse counter, will be affected by tolerances of the used components, the layout of the PCB, temperature effects etc., causing measurement errors. A higher accuracy could be achieved using a temperature-compensated oscillator (TCO), or, in the direct method, by using the synchronization pulse provided by a GPS receiver to time the measurement interval.
\section{Analog Signal Acquisition}\label{sec:theory-adc}
\section{Analog Signal Acquisition}\label{sec:theory-adc}
\todo[inline]{Explain ADC operation and basic concepts / types}
A very common need in experiments involving the measurement of physical properties is the acquisition of analog signals, respective voltages. Those can be roughly divided into DC and time-changing signals. Analog signals are converted to digital values using ADCs (Analog to Digital Converters). Several principles of analog signal measurement exist with different cost, speed, resolution, and many other factors which determine their suitability for a particular application.
DC signals can be measured by taking several samples and calculating their average value; In the presence of a 50\,Hz or 60\,Hz mains interference, its advisable to spread those samples over the 20\,ms (resp. 16.7\,ms) time of one period so that the interfering waveform cancels out. Time-changing signals can be captured by taking isochronous samples at a frequency conforming to the Nyquist theorem, that is, at least twice that of the measured signal. In practice, a frequency several times higher is preferred for a more accurate capture.
\begin{figure}
\centering
\includegraphics[scale=1]{img/sar-adc.pdf}
\caption{\label{fig:adc-sar}A diagram of the SAR type ADC}
\end{figure}
The ADC type commonly available in microcontrollers, including our STM32F072, uses a \textit{successive approximation} method (it is often called the SAR type ADC, after its main component, the \textit{successive approximation register}). Its diagram is shown in figure \ref{fig:adc-sar}.
The SAR type converter uses a DAC (Digital to Analog Converter) which approximates the input voltage, bit by bit, starting from the MSB, following the algorithm outlined below.
\begin{enumerate}
\item The SAR is cleared to all zeros.
\item The DAC generates an approximation voltage.
\item Its output is compared with the sampled input, and the comparator's output is stored as the active bit in the approximation register.
\item The approximation continues with step 2 and the following (less significant) bit.
\item When all bits of the data word were found, an interrupt request is generated and the application program can read it from the SAR.
\end{enumerate}
A change of the input value would make this principle unreliable, which is why the input is buffered by a sample \& hold circuit. The holding capacitor is charged to the input voltage and maintains this level during the conversion. The duration for which the capacitor is connected to the input is called a \textit{sampling time}.
@ -64,10 +85,10 @@ The DAC peripheral can produce a DC level on the output pin based on a control w
A straightforward implementation of the waveform generator is illustrated in figure \ref{fig:wavegen-naive}. This approach has its advantages: it's simple and works entirely in the background, with no interrupt handling required. It could even be implemented entirely in software, using a loop periodically updating the DAC values, of course such approach is less flexible and we would run into problems with asynchronous interrupts.
A straightforward implementation of the waveform generator is illustrated in figure \ref{fig:wavegen-naive}. This approach has its advantages: it's simple and works entirely in the background, with no interrupt handling required. It could even be implemented entirely in software, using a loop periodically updating the DAC values, of course such approach is less flexible and we would run into problems with asynchronous interrupts.
\caption{\label{fig:wavegen-naive}A simple implementation of the waveform generator}
\caption[A simple implementation of the waveform generator]{\label{fig:wavegen-naive}A simple implementation of the waveform generator, using DMA and a look-up table}
\end{figure}
\end{figure}
The highest achievable output frequency largely depends on the size of our look-up table. For instance, assuming a timer frequency of 48\,MHz and a 8192-word table, holding one period of the waveform, the maximum frequency would be short of 6\,kHz, whereas if we shorten the table to just 1024 words, we can get almost 47\,kHz on the analog output. The downside of a shorter table is a lower resolution, which will appear as DC plateaus or steps when observed with an oscilloscope, producing harmonic components similar to those of a square wave.
The highest achievable output frequency largely depends on the size of our look-up table. For instance, assuming a timer frequency of 48\,MHz and a 8192-word table, holding one period of the waveform, the maximum frequency would be short of 6\,kHz, whereas if we shorten the table to just 1024 words, we can get almost 47\,kHz on the analog output. The downside of a shorter table is a lower resolution, which will appear as DC plateaus or steps when observed with an oscilloscope, producing harmonic components similar to those of a square wave.
@ -81,7 +102,7 @@ There are situations where the simple waveform generation method is not sufficie
\begin{figure}[h]
\begin{figure}[h]
\centering
\centering
\includegraphics[scale=1]{img/wavegen-dds.pdf}
\includegraphics[scale=1]{img/wavegen-dds.pdf}
\caption{\label{fig:wavegen-dds}A block diagram of a direct digital synthesis waveform generator}
\caption{\label{fig:wavegen-dds}A block diagram of a DDS-based waveform generator}
\end{figure}
\end{figure}
A diagram of a possible DDS implementation in the STM32 firmware is shown in figure \ref{fig:wavegen-dds}. It is based on a \textit{numerically controlled oscillator} (NCO). NCO consists of a \textit{phase accumulator} register and a \textit{tuning word} which is periodically added to it at a constant rate in a timer interrupt handler. The value of the tuning word determines the output waveform frequency. The look-up table must have a power-of-two length so that it can be addressed by the \textit{n} most significant bits of the phase accumulator. An additional control word could be added to this address to implement a phase offset for applications like a phase-shift modulation.
A diagram of a possible DDS implementation in the STM32 firmware is shown in figure \ref{fig:wavegen-dds}. It is based on a \textit{numerically controlled oscillator} (NCO). NCO consists of a \textit{phase accumulator} register and a \textit{tuning word} which is periodically added to it at a constant rate in a timer interrupt handler. The value of the tuning word determines the output waveform frequency. The look-up table must have a power-of-two length so that it can be addressed by the \textit{n} most significant bits of the phase accumulator. An additional control word could be added to this address to implement a phase offset for applications like a phase-shift modulation.
@ -92,22 +113,19 @@ The output frequency is calculated as \(f_\mathrm{out} = \dfrac{M\cdot f_\mathrm
DDS may be implemented in hardware, including the look-up table, often together with the DAC itself, which is then called a \textit{Complete DDS}. That is the case of e.g. AD9833 from Analog Devices. As the software implementation depends on a periodic interrupt, it's often advantageous to use a component like this when we need higher output frequencies where the use of an interrupt is not possible. GEX can control an external waveform generator like the AD9833 using an SPI port.
DDS may be implemented in hardware, including the look-up table, often together with the DAC itself, which is then called a \textit{Complete DDS}. That is the case of e.g. AD9833 from Analog Devices. As the software implementation depends on a periodic interrupt, it's often advantageous to use a component like this when we need higher output frequencies where the use of an interrupt is not possible. GEX can control an external waveform generator like the AD9833 using an SPI port.
\todo[inline]{screenshots of a real demo, also reference to all-about-direct-digital-synthesis.pdf}
\section{Touch Sensing}\label{sec:theory-touch}
\section{Touch Sensing}\label{sec:theory-touch}
The used microcontroller, STM32F072, includes a touch sensing controller (TSC) peripheral block. It can be accessed from GEX as a demonstration of capacitive touch sensing, and could possibly be used for simple touch sensors as well, such as measuring the level of water in a tank.
The used microcontroller, STM32F072, includes a touch sensing controller (TSC) peripheral block. It can be accessed from GEX as a demonstration of capacitive touch sensing, and could possibly be used for simple touch sensors as well, such as measuring the level of water in a tank.
\begin{figure}[h]
The TSC requires a specific topology with a sampling capacitor connected close to the microcontroller pin, which may not be possible on a universal GEX module; for this reason, the touch sensing feature is best demonstrated on the STM32F072 Discovery development kit, which includes a 4-segment touch slider shown in figure \ref{fig:disco-touch}.
\caption{\label{fig:disco-touch}The touch slider on a STM32F072 Discovery board}
\caption{\label{fig:disco-touch}The touch slider on a STM32F072 Discovery board}
\end{figure}
\end{figure}
The TSC requires a specific topology with a sampling capacitor connected close to the microcontroller pin, which may not be possible on a universal GEX module; for this reason, the touch sensing feature is best demonstrated on the STM32F072 Discovery development kit, which includes a 4-segment touch slider shown in figure \ref{fig:disco-touch}.
The principle of capacitive touch sensing is well explained in the microcontroller's reference manual XXX\todo{ref}. A key part of the TSC is a set of analog switches, which can be combined to form several different signal paths between the external pins, Vdd, GND, and an analog comparator. Two input pins are needed for every touch sensing channel: the sensing pad connects to one, the other is connected through a sampling capacitor (47\,nF on the Discovery board) to GND.
The principle of capacitive touch sensing is well explained in the microcontroller's reference manual \todo{ref}. A key part of the TSC is a set of analog switches, which can be combined to form several different signal paths between the external pins, Vdd, GND, and an analog comparator. Two input pins are needed for every touch sensing channel: the sensing pad connects to one, the other is connected through a sampling capacitor (47\,nF on the Discovery board) to GND.
Capacitive sensing is a sequential process described in the following steps:
Capacitive sensing is a sequential process described in the following steps:
Prototyping, design evaluation and the measurement of physical properties in experiments make a daily occurrence in the engineering praxis. Those tasks typically involve the generation and sampling of electrical signals coming to and from sensors, actuators, and other circuitry.
Prototyping, design evaluation, and the measurement of physical properties in experiments make a daily occurrence in the engineering praxis. Those tasks often involve the generation and sampling of electrical signals coming to and from sensors, actuators, and other circuitry.
In the recent years a wide range of intelligent sensors became available thanks to the drive for miniaturization in the consumer electronics industry. Those devices often provide a sufficient accuracy and precision while keeping the circuit complexity and cost low. In contrast to analog sensors, here the signal conditioning and processing circuits are built into the sensor itself and we access it using a digital connection.
In the recent years, a wide range of intelligent sensors became available thanks to the drive for miniaturization in the consumer electronics industry. Those devices often provide a sufficient accuracy and precision while keeping the circuit complexity and cost low. In contrast to analog sensors, here the signal conditioning and processing circuits are built into the sensor itself and we access it using a digital connection.
@ -32,17 +34,15 @@ We've covered some more advanced features, but skipped the simplest feature: a d
Some sensors have a variable frequency or a pulse-width modulated (PWM) output. To capture those signals and convert them to a more useful digital value, we can use the external input functions of a timer/counter in the microcontroller. Those timers have many possible configurations and can also be used for pulse counting or a pulse train generation.
Some sensors have a variable frequency or a pulse-width modulated (PWM) output. To capture those signals and convert them to a more useful digital value, we can use the external input functions of a timer/counter in the microcontroller. Those timers have many possible configurations and can also be used for pulse counting or a pulse train generation.
\section{Connection to the Host Computer}
\section{Host Computer Connection}
\subsection{Messaging}
\subsection{Communication Interface}
USB shall be the primary way of connecting the module to a host PC. Thanks to USB's flexibility, it can present itself as any kind of device or even multiple devices at once.
USB shall be the primary way of connecting the module to a host PC. Thanks to USB's flexibility, it can present itself as any kind of device or even multiple devices at once.
The most straightforward method of interfacing the board is by passing binary messages in a fashion similar to USART (and plain UART can be available as well). We'll need a duplex connection to enable command confirmations, query-type commands and asynchronous event reporting.
The most straightforward method of interfacing the board is by passing binary messages in a fashion similar to USART (and plain UART can be available as well). We'll need a duplex connection to enable command confirmations, query-type commands and asynchronous event reporting. This is possible either using a "Virtual COM port" driver (the CDC/ACM USB class), or through a raw access to the corresponding USB endpoints. Using a raw access avoids potential problems with the operating system's driver interfering or not recognizing the device correctly; on the other hand, having GEX appear as a serial port makes it easier to integrate into existing platforms that have a good serial port support (such as National Instruments LabWindows CVI or MATLAB).
This is possible either using a "Virtual COM port" driver (the CDC/ACM USB class), or through a raw access to the corresponding USB endpoints. Using a raw access avoids potential problems with the operating system's driver interfering or not recognizing the device correctly; on the other hand, having GEX appear as a serial port makes it easier to integrate into existing platforms that have a good serial port support (such as National Instruments LabWindows CVI or MATLAB).
A wireless attachment is also planned; after establishing a connection, the two-way link should work in a similar manner to UART or USB. \todo[inline]{link to where this is better explained}
A wireless attachment is also planned; after establishing a connection, the two-way radio link should work transparently, in a similar manner to the UART or USB connection.
\subsection{Configuration Files}
\subsection{Configuration Files}
@ -50,9 +50,9 @@ The module must be easily reconfigurable. Given the settings are almost always g
We can load those settings into GEX using the serial interface, which also makes it possible to reconfigure it remotely when the wireless connection is used. With USB, we can additionally make the board appear as a mass storage device and expose the configuration as text files. This approach, inspired by ARM mbed's mechanism for flashing firmware images to development kits, avoids the need to create a configuration GUI, instead using the PC OS's built-in applications like File Explorer and Notepad. We can expose additional information, such as a README file with instructions or a pin-out reference, as separate files on the virtual disk.
We can load those settings into GEX using the serial interface, which also makes it possible to reconfigure it remotely when the wireless connection is used. With USB, we can additionally make the board appear as a mass storage device and expose the configuration as text files. This approach, inspired by ARM mbed's mechanism for flashing firmware images to development kits, avoids the need to create a configuration GUI, instead using the PC OS's built-in applications like File Explorer and Notepad. We can expose additional information, such as a README file with instructions or a pin-out reference, as separate files on the virtual disk.
\section{Planned Feature List}
\section{An Overview of Planned Features}
Let's list the features we wish to initially support in the GEX firmware:
Let's now summarize the features we wish to support in the GEX firmware, based on the preceding discussion:
\begin{itemize}
\begin{itemize}
\item\textbf{Hardware interfacing functions}
\item\textbf{Hardware interfacing functions}
@ -66,8 +66,9 @@ Let's list the features we wish to initially support in the GEX firmware:
\item Dallas 1-Wire
\item Dallas 1-Wire
\item NeoPixel (addressable LED strips)
\item NeoPixel (addressable LED strips)
\end{itemize}
\end{itemize}
\pagebreak[0]
\item\textbf{Communication with the host computer}
\item\textbf{Communication with the host computer}
\begin{itemize}
\begin{itemize}
\item USB connection as virtual serial port or direct endpoint access
\item USB connection as virtual serial port or direct endpoint access
\item Connection using plain UART
\item Connection using plain UART
\item Wireless attachment
\item Wireless attachment
@ -82,7 +83,7 @@ Let's list the features we wish to initially support in the GEX firmware:
\section{Microcontroller Selection}
\section{Microcontroller Selection}
The STM32F072 microcontroller was chosen for the built prototypes and the initial firmware, owning to it's low cost, advanced peripherals and the availability of development boards. GEX can be later ported to other MCUs, like the STM32L072, STM32F103 or STM32F303.
As discussed in section \ref{sec:expected-outcome}, this project will be based on microcontrollers from the STM32 family. The STM32F072 model was selected for the initial hardware and firmware design due to it's low cost, advanced peripherals and the availability of development boards. The firmware can be ported to other MCUs later (e.g. to STM32L072, STM32F103 or STM32F303).
The STM32F072 is a Cortex M0 device with 128\,KiB of flash memory, 16\,KiB of RAM and running at 48\,MHz. It is equipped with a USB Full Speed peripheral block, a 12-bit ADC and DAC, a number of general purpose timers/counters, SPI, I$^2$C, and USART peripherals, among others. It supports crystal-less USB, using the USB SOF packet for synchronization of the internal 48\,MHz RC oscillator; naturally, a real crystal resonator will provide better timing accuracy.
The STM32F072 is a Cortex M0 device with 128\,KiB of flash memory, 16\,KiB of RAM and running at 48\,MHz. It is equipped with a USB Full Speed peripheral block, a 12-bit ADC and DAC, a number of general purpose timers/counters, SPI, I$^2$C, and USART peripherals, among others. It supports crystal-less USB, using the USB SOF packet for synchronization of the internal 48\,MHz RC oscillator; naturally, a real crystal resonator will provide better timing accuracy.
@ -90,21 +91,16 @@ To effectively utilize the time available for this work, only the STM32F072 firm
\section{Form Factor Considerations}
\section{Form Factor Considerations}
It was mentioned in \ref{sec:expected-outcome} that, while the GEX firmware can be used with existing evaluation boards from ST Microelectronics (figure \ref{fig:discovery}), we wish to design and realize a few custom hardware prototypes that will be smaller, more convenient to use and hopefully also cheaper. Three possible form factors are drawn in figure \ref{fig:ff-sketches}.
While the GEX firmware can be used with existing evaluation boards from ST Microelectronics (see figure \ref{fig:discovery} for an example of one such board), we wish to design and realize a few custom hardware prototypes that will be smaller and more convenient to use.
Several factors play a role when deciding what the GEX PCB should look like:
Three possible form factors are drawn in figure \ref{fig:ff-sketches}. The use of a common connector layout and pin assignments, here Arduino and Raspberry Pi, makes it possible to reuse add-on boards from those platforms. When we copy the physical form factor of another product, in this example the Raspberry Pi Zero, we can further take advantage of existing enclosures designed for it.
The device must be comfortable and easy to use, which affects the choice of the USB connector, also with respect to cable availability: USB type A is not suitable for desktop computers where it would have to be plugged in the rear of the computer or in the front panel, but it may be usable with laptops; USB Mini-B and Micro-B connectors are both a popular choice in existing kits (e.g. Discovery and Nucleo boards), but Micro-B has a higher rated number of insertions and the cables are ubiquitous thanks to their use in mobile phones, therefore this appears to be the better connector choice. \todo{link to the insertion count spec}
The PCB size should be kept minimal to save manufacturing costs. When a standard connector shape and a pin assignment are used we gain the ability to install existing add-on boards designed for other platforms, like the Arduino or Raspberry Pi. Lastly, when the entire board shape is copied from an existing commercial product for which we can buy official or after-market cases, we get an easy access to cases without having to design them ourselves. This is the case of the Raspberry Pi Zero form factor.
The digital/analog unit works with the two-channel DAC peripheral of the microcontroller. It can be used in two modes: DC output, and waveform generation.
The digital/analog unit works with the two-channel DAC peripheral of the microcontroller. It can be used in two modes: DC output, and waveform generation.
The waveform mode implements direct digital synthesis (explained in section \ref{sec:theory-dac-dds}) of a sine, rectangle, sawtooth or triangle wave. The generated frequency can be set with a sub-hertz precision up to the lower tens of kHz. The two outputs can use a different waveform shape, be synchronized, and their phase offset, as well as frequency, is dynamically adjustable.
The waveform mode implements direct digital synthesis (explained in section \ref{sec:theory-dac-dds}) of a sine, rectangle, sawtooth or triangle wave. The generated frequency can be set with a sub-hertz precision up to the lower tens of kHz. The two outputs can use a different waveform shape, be synchronized, and their phase offset, as well as frequency, is dynamically adjustable.
\subsection{DAC Configuration}
\begin{inicode}
[DAC:dac@13]
# Enabled channels (1:A4, 2:A5)
ch1_enable=Y
ch2_enable=Y
# Enable output buffer
ch1_buff=Y
ch2_buff=Y
# Superimposed noise type (NONE,WHITE,TRIANGLE) and nbr. of bits (1-12)
ch1_noise=NONE
ch1_noise-level=3
ch2_noise=NONE
ch2_noise-level=3
\end{inicode}
\subsection{DAC Commands}
Channels are specified in all commands as bit map:
\begin{itemize}[nosep]
\item 0x01 - channel 1
\item 0x02 - channel 2
\item 0x03 - both channels affected at once
\end{itemize}
\begin{cmdlist}
0 &\cname{WAVE\_DC}
Set a DC level, disable DDS for the channel
&\begin{cmdreq}
\cfield{u8} channels
\cfield{u16} level (0--4095)
\end{cmdreq}\\
1 &\cname{WAVE\_SINE}
Start a DDS sine waveform
&\begin{cmdreq}
\cfield{u8} channels
\end{cmdreq}\\
2 &\cname{WAVE\_TRIANGLE}
Start a symmetrical triangle waveform
&\begin{cmdreq}
\cfield{u8} channels
\end{cmdreq}\\
3 &\cname{WAVE\_SAWTOOTH\_UP}
Start a rising sawtooth waveform
&\begin{cmdreq}
\cfield{u8} channels
\end{cmdreq}\\
4 &\cname{WAVE\_SAWTOOTH\_DOWN}
Start a dalling sawtooth waveform
&\begin{cmdreq}
\cfield{u8} channels
\end{cmdreq}\\
5 &\cname{WAVE\_RECTANGLE}
Start a rectangle waveform
&\begin{cmdreq}
\cfield{u8} channels
\cfield{u16} on-time (0--8191)
\cfield{u16} high level (0--4095)
\cfield{u16} low level (0--4095)
\end{cmdreq}\\
10 &\cname{SYNC}
Synchronize the two channels. The phase accumulator is reset to zero.
&\\
20 &\cname{SET\_FREQUENCY}
Set the channel frequency
&\begin{cmdreq}
\cfield{u8} channels
\cfield{float32} frequency
\end{cmdreq}\\
21 &\cname{SET\_PHASE}
Set a channel's phase. It's recommended to set the phase only of one channel, leaving the other at 0°.
&\begin{cmdreq}
\cfield{u8} channels
\cfield{u16} phase (0--8191)
\end{cmdreq}\\
22 &\cname{SET\_DITHER}
Control the dithering function of the DAC block. A high noise amplitude can cause an overflow to the other end of the output range due to a bug in the DAC peripheral. Use value 255 to leave the parameter unchanged.
&\begin{cmdreq}
\cfield{u8} channels
\cfield{u8} noise type (0-none, 1-white, 2-triangle)
The touch sensing unit provides an access to the touch sensing controller. Its function is explained in section \ref{sec:theory-touch}. The unit configures the TSC and reads the output values of each enabled touch pad.
The touch sensing unit provides an access to the touch sensing controller. Its function is explained in section \ref{sec:theory-touch}. The unit configures the TSC and reads the output values of each enabled touch pad.
Four methods of a wireless connection have been considered: Bluetooth (e.g. with CC2541), WiFi with ESP8266, LoRA or GFSK-modulated 868\,MHz link with SX1276, and a 2.4\,GHz link with nRF24L01+. Bluetooth was dismissed early for its complexity and ESP8266 for its high consumption in continuous reception mode, although both solutions might be viable for certain applications and with more time for evaluation.
Four methods of a wireless connection have been considered: Bluetooth (e.g. with CC2541), WiFi with ESP8266, LoRA or GFSK-modulated 868\,MHz link with SX1276, and a 2.4\,GHz link with nRF24L01+. Bluetooth was dismissed early for its complexity and ESP8266 for its high consumption in continuous reception mode, although both solutions might be viable for certain applications and with more time for evaluation.
