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101 lines
6.8 KiB
101 lines
6.8 KiB
\chapter{Hardware Realization} \label{sec:hwreal}
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\section{Using a Discovery Board}
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It has been proposed earlier in the text that STM32 Nucleo and Discovery development boards might be used as the hardware platform for this project. Indeed, a Discovery board with STM32F072~\cite{disco-f072} was used as a development platform for the majority of the GEX firmware. This inexpensive board may be used to try the GEX firmware without having access to the custom hardware.
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\subsection{Discovery STM32F072 Configuration and Pin Mapping}
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This Discovery board is fitted with four \glspl{LED} on \gls{GPIO} pins PC6 through PC9, in a compass arrangement. The ``north'' \gls{LED}, PC6, is used as the GEX status indicator. The ``User'' button, connected to PA0, is mapped as the GEX Lock button, controlling the settings storage.
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We advise the reader, as a potential user of this discovery board, to review its schematic diagram and ensure the solder-jumpers are configured correctly:
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\begin{itemize}
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\item Jumpers SB20 and SB23 must be closed to enable the User \gls{USB} connector.
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\item Jumper SB17 must be open and SB19 closed to use the 8\,MHz clock signal provided by the on-board ST-Link programmer; the internal USB-synchronized 48\,MHz oscillator will be used if the clock signal is not provided (SB19 open).
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\item Jumpers SB27 through SB32 should be closed to connect the \gls{GPIO} pins normally dedicated to the touch sensing strip to the board's header.
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\item Capacitors C26 through C28 are sampling capacitors for the \gls{TSC}. There are, unfortunately, no jumpers available to disconnect them, and they interfere in high-speed signals on the used pins (PA3, PA7, PB1). The only solution is to carefully remove them from the board if the \gls{TSC} is not needed.
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\end{itemize}
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An accelerometer \gls{IC} L3GD20 is fitted on the board. The chip is attached to SPI2 on pins PB13 (\gls{SCK}), PB14 (\gls{MISO}) and PB15 (\gls{MOSI}), with \gls{NSS} on pin PC0, and PC1 and PC2 used for interrupt flags. This chip cannot be disconnected or disabled and it is difficult to remove; care must be taken to avoid its interference on the used pins.
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\section{GEX Hub}
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GEX Hub was the first custom \gls{PCB} designed for GEX. It uses the same microcontroller as the Discovery board, thus the firmware modifications needed to make it work with this new platform were minimal.
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The Hub board provides access to all the \gls{GPIO} pins using three flat-cable connectors, one for each port; they also contain a ground and power supply connection to make the connection of external boards or a breadboard easier, needing just one cable. The use of flat cables, however, is not mandatory---those connectors are based on the standard 2.54\,mm pitch pin headers, allowing the user to connect to them using widely available ``jumper wires''.
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This board was produced in two revisions. The original model ( \cref{fig:gexhub1}) proved fully functional, except for the two connectors on the left side, the boot jumper and a programming header, which had the wrong footprints and could not be populated; this mistake was fixed by soldering the jumper from the bottom of the \gls{PCB}, and the programming header was never needed thanks to the USB bootloader working without issues.
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The updated revision removes the two problematic footprints altogether; a reorganization in the \gls{GPIO} connectors allowed them to be moved together with the other pins. Revision 1 used a dedicated header for the Boot jumper that was meant to be closed during normal operation, and removed only to enter the bootloader. Revision 2 moved the boot pin into the connector, and such arrangement would not be practical; the solution was to invert the jumper's logic by changing the Boot pull-up to a pull-down. The bootloader is now activated by inserting a regular 2.54\,mm jumper into the connector\footnote{A restart is required in all cases for the boot jumper changes to have effect}, as can be seen in \cref{fig:gexhub2}.
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\begin{figure}[h]
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\centering
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\begin{subfigure}{.5\textwidth}
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\centering
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\includegraphics[width=.98\linewidth]{img/photo-hub1.jpg}
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\caption{\label{fig:gexhub1}Revision 1}
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\end{subfigure}%
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\begin{subfigure}{.5\textwidth}
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\centering
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\includegraphics[width=.98\linewidth]{img/photo-hub2.jpg}
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\caption{\label{fig:gexhub2}Revision 2}
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\end{subfigure}
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\caption[The GEX Hub module]{\label{fig:gexhub} Two revisions of the GEX Hub module, rev. 2 shown with the boot jumper and a flat cable.}
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\end{figure}
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\section{GEX Zero}
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Our desire to re-use the form factor of the Raspberry Pi Zero to exploit the existing market with add-on boards and cases for it has been mentioned already in \cref{sec:formfactors}. It was brought to fruition with GEX Zero, the second realized prototype board (counting the two revisions of GEX Hub as one).
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GEX Zero exactly copies the dimensions of the Pi Zero, which introduces several challenges:
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\begin{itemize}
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\item It must be a one-sided board, with no components on the bottom; this is needed for acrylic cases which sit flatly against the \gls{PCB}, with a cut-out for the pin header.
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\item Buttons and the USB connector have to exactly align with connectors on the Pi Zero to fit the openings in its cases.
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\item The board size is fixed, and rather small; we used only two layers to save production cost, but this proved a significant challenge and the electrical characteristics of some connections may not be ideal.
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\item To make use of the Raspberry Pi add-on boards, called HATs or pHATs, a particular organization of the pin header is required. This is discussed in more detail below.
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\end{itemize}
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\subsection{Finding the Best Pin Assignment}
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Like our STM32 microcontroller, the Broadcom processor on the Raspberry Pi multiplexes its \gls{GPIO} pins with alternate functions, and, likewise, each function is available only on a small selection of pins. A number of compromises had to be made to achieve maximum compatibility.
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\todo[inline]{show the pi header mappings and the gex zero pin mappings}
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\begin{figure}[h]
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\centering
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\includegraphics[width=.9\textwidth]{img/photo-zero-naked.jpg} \\
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\vspace{1mm}
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\includegraphics[width=.9\textwidth]{img/photo-zero-naked-bottom.jpg}
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\caption[The GEX Zero module]{\label{fig:gexz}GEX Zero, top and bottom side}
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\end{figure}
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\begin{figure}[h]
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\centering
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\includegraphics[width=.9\textwidth]{img/photo-zero-picase.jpg} \\
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\vspace{1mm}
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\includegraphics[width=.9\textwidth]{img/photo-zero-transparent.jpg}
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\caption[The GEX Zero module]{\label{fig:gexzcases}GEX Zero in the official Raspberry Pi Zero case and an aftermarket acrylic case}
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\end{figure}
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\section{Wireless Gateway} \label{sec:rfgateway}
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\cref{fig:gwxgw}
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\todo[inline]{TODO about the gateway ..}
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\begin{figure}[h]
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\centering
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\includegraphics[width=.9\textwidth]{img/photo-rfdongle.jpg}
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\caption{\label{fig:gwxgw}The wireless gateway module (top and bottom side)}
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\end{figure}
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