STM32-Tx-HID
------------

This project aims to bring inexpensive and easy solution to the task
of connecting various RC transmitters to a computer.

With the wide availability of relatively cheap RC toys a beginner user
might end up with a reasonably decent collective pitch helicopter and
a cheap transmitter as part of an RTF bundle. Learning to control a CP
heli can be a daunting task, so a lot of people prefer to practice
with a computer simulation. However, many transmitters do not support
USB connectivity (and some do not even provide a PPM output). To
connect equipment like that to a computer, additional hardware is
needed.

This firmware is targetting the cheapest STM32F103 board as found in
numerous listings on eBay and Aliexpress (might have the following
keywords: ARM STM32F103C8T6 Minimum System Development Board Module
For Arduino).

The board photo is in stm32f103-board.jpg.

Once the firmware is installed and running, a computer will see a
regular HID joystick with 8 analog channels and 8 buttons. During USB
operation the power switch must be in the off state, the device is
bus-powered. Once you disconnect the board from a computer and power
on the transmitter normally, the board goes to a sleep mode and
doesn't interfere with normal operation.

* Board description
The board has an STM32F103C8T6 controller (in LQFP48 package),
microusb connector, 8 MHz main quartz and an additional 32768 kHz
quartz for clock. The controller is powered from an LDO outputting
3.3V from the 5V USB bus. D+ pullup resistor is permanently attached
but software can force USB bus reset by putting the corresponding pin
(PA12) in GPIO push-pull mode and setting it low for at least
2.5us. It also has a physical reset button and two LEDs: one
permanently on, one controlled by PC13 (active low).

In addition to that there're 4 pins exposed for SWD (flashing and
debug) and two jumpers (unlabeled). Both jumpers can connect their
middle pin either to the GND ("0" position) or the Vcc ("1"
position). The jumper that is closer to the board side has middle pin
connected to BOOT0 via a 100k series resistor and the other jumper is
connected via a 100k series resistor to BOOT1.

For reliable board startup both BOOTx pins should be pulled low ("0"
position).

* Building the firmware
After cloning the repository "git submodule init; git submodule
update" needs to be run once, then "make" to build everything
necessary.

It is assumed that "arm-none-eabi" toolchain is available in PATH,
"GCC ARM Embedded" is the recommended option. In case you need to
specify a path, CROSS_COMPILE make variable should be set
appropriately.

* Initial flashing procedure
There're two ways of flashing a blank board: via SWD and via serial
bootloader. For the first method a suitable debug adapter is needed
(e.g. ST-Link or J-Link), for the second one can use just about any
USB-UART bridge (the only requirement that is a bit unusual is to
support "even" parity).

The image to be flashed is "stm32-tx-hid-bootldr-combined.bin", it
contains both a DFU-compliant bootloader and the main firmware. After
the bootloader is in place, you can reflash via USB without
disassembly and attaching any additional hardware, see the next
section.

To flash the controller via its serial ROM bootloader the BOOT0 jumper
(the one closer to the board side) should be switched to "1" and
USART1 (PA9/TX, PA10/RX) attached to a computer running stm32flash
software (http://sourceforge.net/projects/stm32flash/, alternatively,
windows users might consider
http://www.st.com/web/en/catalog/tools/PF257525). When flashing the
board for the first time full mass-erase might need to be performed by
using stm32flash's "-k" option ("Disable the flash
read-protection"). Do not forget to switch the jumper back to "0"
after flashing is complete.

To flash via SWD with ST-Link the following command can be used:
openocd -f interface/stlink-v2.cfg -f target/stm32f1x.cfg \
	-c "init; reset halt; stm32f1x mass_erase 0" \
	-c "program stm32-tx-hid-bootldr-combined.bin 0x08000000 verify reset exit"

* Reflashing via the DFU bootloader
If the main firmware is functioning normally, dfu-util will be able
to reset the board into the bootloader mode automatically. In case
that doesn't work one can force bootloader mode by pulling both PB8
and PB9 low before attaching USB.

To update the main board firmware use the following command:

dfu-util -R -s 0x08002000 -D stm32-tx-hid.bin

Alternatively, windows users can consider ST's DfuSe utility
(http://www.st.com/web/en/catalog/tools/FM147/CL1794/SC961/SS1533/PF257916).

* Connecting the board to the transmitter
The firmware assumes the following connections are made:

1. PA0-PA7 are used as 8 analog channels; ground the unused ones;
2. PB8-PB15 are used as 8 discrete inputs, pulled-up internally;
3. PB0 is pulled low when USB is connected, it might be attached to
   the transmitter's microcontroller's nRST line to stop it from
   interfering;
4. GND and 3.3V must be connected to the corresponding transmitter
   points.

* Development and contacts
The development repository is hosted at:
https://github.com/paulfertser/stm32-tx-hid.

If you have any suggestions or questions, feel free to contact me
directly via email.

Have fun, and happy flying,
Paul Fertser <fercerpav@gmail.com>