@ -101,7 +101,7 @@ The FAT16 file system is relatively straightforward to implement. However, it is
\subsection{DAPLink Emulator}
A FAT16 emulator was developed as part of the open-source Arm Mbed DAPLink project~\cite{daplink}. It is used there for a drag-and-drop flashing of firmware images to the target microcontroller, taking advantage of the inherent cross-platform support (it uses the same software driver as any thumb drive, as discussed in \cref{sec:msc}). Arm Mbed also uses a browser-based \gls{IDE} and cloud build servers, thus the end user does not need to install or set up any software to program a compatible development kit.
A FAT16 emulator was developed as part of the open-source \mbed DAPLink project~\cite{daplink}. It is used there for a drag-and-drop flashing of firmware images to the target microcontroller, taking advantage of the inherent cross-platform support (it uses the same software driver as any thumb drive, as discussed in \cref{sec:msc}). \mbed also uses a browser-based \gls{IDE} and cloud build servers, thus the end user does not need to install or set up any software to program a compatible development kit.
The GEX firmware adapts several parts of the DAPLink code, optimizing its \gls{RAM} usage and porting it to work with FreeRTOS. Those modified files are located in the folder \mono{User/vfs} of the GEX source code repository; the original Apache 2.0 open source software license headers, as well as file names, have been retained.
@ -18,7 +18,7 @@ Tasks can be in one of four states: Suspended, Ready, Blocked, Running. The Susp
\subsubsection{Task Switching and Interrupts}
Task switching occurs periodically in a timer interrupt, usually every 1\,ms; in Cortex-M, this is typically the SysTick interrupt, a timer designed for this purpose that is included in the Arm core itself and thus available on all derived platforms.
Task switching occurs periodically in a timer interrupt, usually every 1\,ms; in \armcm chips this is typically the SysTick interrupt, a timer designed for this purpose that is included in the core itself and thus available on all derived platforms.
After one tick of run time, the Running task is paused (enters Ready state), or continues to run if no higher priority task is available. If a higher priority task waits for an object and this is made available in an interrupt, the previously running task is paused and the waiting task is resumed immediately (enters the Running state). FreeRTOS defines an interrupt-friendly variant of some of the \gls{API} functions intended for this purpose.
@ -204,7 +204,7 @@ The framework provides one more messaging service to the units: event reporting.
Interrupts are an important part of almost any embedded application. They provide a way to rapidly react to asynchronous external or internal events, temporarily leaving the main program, jumping to an interrupt handler routine, and then returning back after the event is handled. Interrupts are also the way FreeRTOS implements multitasking without a multi-core processor.
In the Cortex-M0-based STM32F072, used in the initial GEX prototypes, the interrupt handlers table, defining which routine is called for which interrupt, is stored in the program memory and cannot be changed at run-time. This is a complication for the modular structure of GEX where different unit drivers may use the same peripheral, and we would want to dynamically assign the interrupt handlers based on the active configuration. Let's have a look at an interrupt handler, in this case handling four different \gls{DMA} channels, as is common in STM32 microcontrollers:
In the \armCortex-M0-based STM32F072, used in the initial GEX prototypes, the interrupt handlers table, defining which routine is called for which interrupt, is stored in the program memory and cannot be changed at run-time. This is a complication for the modular structure of GEX where different unit drivers may use the same peripheral, and we would want to dynamically assign the interrupt handlers based on the active configuration. Let's have a look at an interrupt handler, in this case handling four different \gls{DMA} channels, as is common in STM32 microcontrollers:
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.
Recently, a wide range of intelligent sensors became available thanks to the drive for miniaturization in the consumer electronics industry. Those devices often provide 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.
Recently, a wide range of intelligent sensors became available thanks to the drive to miniaturization in the consumer electronics industry. Those devices often provide 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.
\caption[A collection of intelligent sensors and devices]{A collection of intelligent sensors and devices, most on breadboard adapters: (from top left) a waveform generator, a gesture detector, a LoRa and two Bluetooth modules, an air quality and pressure sensor, a CO$_2$ sensor, a digital compass, an accelerometer, a GPS module, a camera, an ultrasonic range finder, a humidity sensor, a 1-Wire thermometer, a color detector and an RGB LED strip.}
\caption[A collection of intelligent sensors and devices]{A collection of intelligent sensors and devices, most on breadboard adapters: (from the top left) a waveform generator, a gesture detector, a LoRa and two Bluetooth modules, an air quality and pressure sensor, a CO$_2$ sensor, a digital compass, an accelerometer, a GPS module, a camera, an ultrasonic range finder, a humidity sensor, a 1-Wire thermometer, a color detector, and an RGB LED strip}
\end{figure}
To conduct experiments with those integrated modules, or just familiarize ourselves with a device before using it in a project, we need an easy way to interact with them. It is also convenient to have direct access to hardware, be it analog signal sampling, generation, or even just logic level inputs and outputs. However, the drive for miniaturization and the advent of\gls{USB} lead to the disappearance of low-level computer ports, such as the printer port (LPT), that would provide an easy way of doing so.
If we wish to conduct experiments with those integrated modules, or just familiarize ourselves with a device before using it in a project, we need an easy way to interact with them. It would also be convenient to have direct access to low-level hardware, be it analog signal sampling, generation, or even just the access to logic inputs and outputs. However, advances in computer technology, namely the advent of the\gls{USB}, lead to the disappearance of low-level computer ports, such as the printer port (LPT), that would provide an easy way of doing so.
Today, when one wants to perform measurements using a digital sensor, the usual route is to implement an embedded firmware for a microcontroller that connects to the \gls{PC} through \gls{USB}, or perhaps shows the results on a display. This approach has its advantages, but is time-consuming and requires knowledge entirely unrelated to the measurements we wish to perform. It would be advantageous to have a way to interface hardware without having to burden ourselves with the technicalities of the connection, even at the cost of lower performance compared to a specialized device or a professional tool.
Today, when we want to perform measurements using a digital sensor, the usual route is to implement an embedded firmware for a microcontroller that connects to the \gls{PC} through \gls{USB}, or perhaps shows the results on a display. This approach has its advantages, but is time-consuming and requires specific knowledge unrelated to the measurements we wish to perform. It would be advantageous to have a way to access hardware without having to burden ourselves with the technicalities of this connection, even at the cost of lower performance compared to specialized devices or professional tools.
The design and implementation of such a universal instrument is the object of this work. For technical reasons, such as naming the source code repositories, we need a name for the project; it'll be hereafter called \textit{GEX}, a name originating from ``GPIO Expander''.
The design and implementation of such a universal instrument is the object of this work. For technical reasons, such as naming the source code repositories, we need a name for the project; it shall be, hereafter, called \textbf{GEX}, a name originating from ``\textbf{G}PIO \textbf{Ex}pander''.
It has been a desire of the author for many years to create a universal instrument connecting low-level hardware to a computer, and, with this project, it is finally being realized. Several related projects approaching this problem from different angles can be found on the internet; those will be presented in \cref{sec:prior-art}. This project should not end with yet another tinkering tool that will be produced in a few prototypes and then forgotten. By building an extensible, open-source platform, GEX can become the foundation for future projects which others can expand, re-use and adapt to their specific needs.
It has been a long-time desire of the author to create a universal instrument connecting low-level hardware to a computer, and, with this project, it is finally being realized. Several related projects approaching this problem from different angles can be found on the internet; some of these will be presented in \cref{sec:prior-art}.
Our project is not meant to end with a tinkering tool that will be produced in a few prototypes and then forgotten. By creating an extensible, open-source platform, GEX can become the foundation for future projects which others can expand, re-use and adapt to their specific needs.
\iffalse
\begin{figure}[H]
@ -28,7 +30,7 @@ It has been a desire of the author for many years to create a universal instrume
\end{figure}
\fi
Building on the experience with earlier embedded projects, an STM32 microcontroller shall be used. Those are ARM Cortex M devices with a wide range of hardware peripherals that appear be a good fit for the project. Low-cost evaluation boards are widely available that could be used as a hardware platform instead of developing a custom \gls{PCB}. Besides, those chips are relatively cheap and already popular in the embedded hardware community; there is a good possibility of the project building a community around it and growing beyond what will be presented in this paper.
Building on the experience with earlier embedded projects, an STM32 microcontroller shall be used. Those are \armcm devices with a wide range of hardware peripherals that appear be a good fit for the project. Low-cost evaluation boards are widely available that could be used as a hardware platform instead of developing a custom \gls{PCB}. STM32 microcontrollers are relatively cheap and already popular in the embedded hardware community; there is a real possibility of the project gathering a community around it and growing beyond what will be presented in this paper.
\iffalse
Besides the use of existing development boards, custom \glspl{PCB} will be developed in different form factors. The possibilities of wireless connection should be evaluated. This feature should make GEX useful for instance in mobile robotics or when installed in poorly accessible locations.
@ -48,7 +48,7 @@ A connection using a hardware \gls{UART} is also planned, as a fallback for boar
The module must be easily reconfigurable. Given the settings are almost always going to be tied on the connected external hardware, it would be practical to have an option to store them permanently in the microcontroller's non-volatile memory.
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 \gls{GUI}, instead using the built-in applications of the \gls{PC}\gls{OS}, 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 \mbed's mechanism for flashing firmware images to development kits, avoids the need to create a configuration \gls{GUI}, instead using the built-in applications of the \gls{PC}\gls{OS}, 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{An Overview of Planned Features}
@ -85,7 +85,7 @@ Let's now summarize the features we wish to support in the GEX firmware, based o
As discussed in \cref{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 its low cost, advanced peripherals, and the availability of development boards. The firmware can be ported to other \glspl{MCU} later (e.g., to STM32L072, STM32F103 or STM32F303).
The STM32F072 is a Cortex M0 device with 128\,KiB of flash memory, 16\,KiB of \gls{RAM} and running at 48\,MHz. It is equipped with a \gls{USB} Full Speed peripheral block, a 12-bit \gls{ADC} and \gls{DAC}, a number of general-purpose timers/counters, SPI, I$^2$C, and USART peripherals, among others. It supports crystal-less \gls{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 an \armcm device with 128\,KiB of flash memory, 16\,KiB of \gls{RAM} and running at 48\,MHz. It is equipped with a \gls{USB} Full Speed peripheral block, a 12-bit \gls{ADC} and \gls{DAC}, a number of general-purpose timers/counters, SPI, I$^2$C, and USART peripherals, among others. It supports crystal-less \gls{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.
To effectively utilize the time available for this work, only the STM32F072 firmware will be developed while making sure the planned expansion is as straightforward as possible.
@ -7,7 +7,7 @@ The unit has several operational modes: idle, reciprocal continuous, reciprocal
\subsection{Value Conversion Formulas}
Several of the features implemented in this unit would require floating point arithmetic to provide the measured value in the desired units (Hz, seconds). That is not available in Cortex-M0, only as a software implementation. The calculation is left to the client in order to save Flash space that would be otherwise used by the arithmetic functions. This arrangement also avoids rounding errors and a possible loss of precision.
Several of the features implemented in this unit would require floating point arithmetic to provide the measured value in the desired units (Hz, seconds). That is not available in \armcm, only as a software implementation. The calculation is left to the client in order to save Flash space that would be otherwise used by the arithmetic functions. This arrangement also avoids rounding errors and a possible loss of precision.
