# STM32 Development Board
This target is a generic STM32 development board. The idea is to make
it easy to create custom transmitters based on those
boards. Hopefully, some of the code for supporting generic hardware
will spin back into the base, making it possible for hardware hackers to
add features without needing a custom build to support them.

# Board requirements and features.

The minimum CPU requirements are an STM32F1, 256K of flash and 48K of
ram. While deviation supports chips with as little as 128K of flash
and 20K of ram, those requires removing features and dynamically
loading code in order to fit, among other issues. Given that we're
picking a board in advance, there's little point in choosing such a
board. At this time, there's not much call for more, though some
functions may want a faster CPU or more flash as build code for more
general configuration.

A UI is required, either built into the dev board or as an extra
module. Existing UI's are all based on LCDs and buttons, though a
touch screen clearly makes the buttons optional. There are GUI's based
on multi-line text displays (which may work on a single-line of text;
something to play with in the future), B&W graphic displays, and color
touch screens. The only initial requirement is that libopencm3 have a
driver for the display and touch screen.

Some form of non-volatile storage is also require above that used to
run code. The exact minimum depends on the display technology, as it
is used to store fonts and graphics for the display. 2MB is
recommended for a B&W display, 4MB for a color screen. There is an
experimental file system for use with very small - 10s of K - chips,
but again this is not something you'd use if given the choice, and the
point is to be given the choice.

Finally, if the CPU has a DAC, then audio playback can in theory be
handled by the CPU, with no need for an external audio module as used
in the transmitters. This is subject to verification, but could well
require a faster CPU, and will definitely require more external storage
for storing audio.

# A survey of STM boards

The general target are the STMicroelectronics evaluation and
development boards. They have three different lines, the "Eval",
"Discovery" and "Nucleo" Boards.

The Eval boards tend to have lots of external circuitry - serial
connectors, etc. - that aren't of use for this project. The ones I
looked at also cost orders of magnitude more than the Discovery and
Nucleo boards, so they were dropped from consideration.

The Discovery and Nucleo boards all have built in ST-LINK programmers
with a USB connection that can be used to load new firmware. The more
recent ones also support the CMSIS-DAP, making them particularly easy
to program.

## Discovery boards
The Discovery boards are meant for prototyping, and as such have
feature sets designed to be inexpensive but flexible. They generally
start at around $30, though the older STM32F4 Discovery and
STM32F3 Discovery are available for about half that if you can find
them. But if you're looking at these, see the Nucleo boards below.

The STM32F0, STM32L0, STM32l1 and STM32VL boards don't have sufficient
flash, so aren't suitable.

The STM32F3 boards are also generally unsuitable due to lack of
memory. The exception is the STM32F3 Discovery. However, it's about
the same price as the STM32F4 Discovery and much less powerful, so
there's little reason to consider it.

The STM32F4 boards also come in a number of lines. The STM32F40x and
STM32F41x are nearly identical, with different sensors and minor CPU
tweaks. Any of them should do just fine. The original STM32F4
Discovery is more than adequate and has a DAC. It's also seen as the
STM32F407Discovery, and newer versions have a fancier ST-LINK that
won't make any difference for this project. The older one can be found
- sometimes - for about $15, includes a DAC but no display or
non-volatile memory. It's probably the least expensive of the
Discovery boards that are adequate for this project.

The STM32F429I and STM32F469I Discovery boards are particularly
interesting, as they both include color touch screens as part of the
package. The 429I is available for that $30 price point, but both of
it's DAC outputs are used to drive the LCD, which makes playing music
through it interesting, to say the least. It's also has no
non-volatile storage. The 469I is still being evaluated, but shouldn't
have that problem, as well as having both non-volatile ram and an SD
card slot. However, it's also twice as expensive.

The STM32L476G Discovery is also interesting. It has a 24 segment LCD
and audio output at that $30 price point. Assuming the 24 segment LCD
can be made to work, it might be the least expensive "complete"
discovery solution.

The STM32F7 kits include enhanced networking. Might be interesting for
experimental projects, but are overkill and overpriced for a
transmitter.

I have an F4-Discovery, 429I and 469I that I'll be testing on.

## Nucleo boards
The Nucleo boards are a later development aimed more at the "maker"
market. The really interesting thing about them is that inside the
expansion headers typical of the Discovery boards, they have
Arduino-compatible expansion headers. These have the same physical
layout as the Arduino Uno, and the pins that are available on each
connector have functionality matching those pins on an Arduino, so you
ought to be able to make use of an Arduino board, though you'll need
your own library. This also means they tend to have fewer onboard
devices, since it's inexpensive to add them. For some reason, the
hardware seems to be a bit cheaper, matching the Arduino quality
instead of the Discovery board quality. Net result is that many of
them are available for around $15. While Discovery boards can be had
for that, it's only the older, less interesting ones.

The most interesting possible addition is a touch screen LCD with an
SD card reader on it. This are available for the Arduino for as little
as $5, though more like $10 from a reputable vendor. Which still means
you can get a system with all the relevant hardware for less than most
of the Discovery boards cost.

They also have the built-in ST-LINK programmer, but it's designed to
be "broken off", and each board can be used stand-alone. That mean you
get an ST-LINK to use for other things, as well as a smaller footprint
on the nucleo board.

The STM32L0, STM32F0, STM32F1 and STM32F3 Nucleo boards are
underpowered. The STM32F2 and STM32F7 boards all have Ethernets and a
higher price tag, so aren't really interesting.

The STM32L4 boards use low-power version of the STM32F4 chips. They
tend to cost a little more and have slower clocks, but are otherwise
fine. Some of them even have an LCD display driver in the chip! They
cost more than their STM32F4 counterparts, but less than Discovery
boards. Might be interesting if you're worried about power
consumption, but I can't see that for this project.

The STM32F4 boards are the most diverse of the lot. The STM32F4##Z
boards all have Ethernet, which more than doubles the price, so aren't
interesting. The STM32F401RB doesn't have enough flash, but the rest -
including the STM32F401RE - are fine, just with more processing power
on board as you go to higher numbers and prices.

I've got an STM32F401RE and an Arduino touch screen LCD + SD card to
test with.

## Other options
There are a wide variety of boards based on the STM32 µ-controllers
available from a number of vendors. There's no reason they couldn't be
use for this build, and some of them may have features that aren't
available on the STM boards. If you find one that's particularly
interesting for some reason or price, let us know!