RPi and STM32F4Discovery

This is a very brief description of how the STM32F4DISCOVERY board, with the addition of a couple of very cheap components, can be used as a Real Time Clock ( RTC ) for the Raspberry Pi. With a little extra effort, you get to use the excellent analog to digital converters, digital to analog converters, SPI, I2C and CAN interfaces - an ideal platform for teaching electronics, programming, automation and control for not much more than £10.

I have used software from the fantastic ChibiOS project which provides a Real Time Operating System ( RTOS ) for this board and many others. In addition to providing the RTOS, the project also provides drivers for the microcontroller peripherals and several example projects, making it very easy to get started.

The project was compiled with tools and a compiler from the Yagarto project.

Hardware

You'll need a fine tipped soldering iron, some solder and these items from Farnell:
STMICROELECTRONICS - STM32F4DISCOVERY - BOARD, EVAL, STM32F4-DISCOVERY Order Code: 2009276EPSON TOYOCOM - MC-306, 32.768KHZ, 6PF - CRYSTAL, MC-306, 32.768KHZ, 6PF Order Code: 1712823
MULTICOMP - MCCA000925 - CAPACITOR, 0603, NP0, 10V, 6.8PF Order Code: 1856077RLPANASONIC - CR-2032/BN - CELL, LITHIUM COIN, 220MAH,3V,2032 Order Code: 5219590
MULTICOMP - CH7410-2032LF - BATTERY HOLDER, SMT, 20MM Order Code: 2064724

If you haven't got suitable wire, you might need some:
PRO POWER - 100-30G - WIRE, KYNAR, 30AWG, GREEN, 100M Order Code: 1202483

If you haven't got suitable USB leads then you might need:
MULTICOMP - SPC20060 - USB 2.0 CABLE ASS, CONNECTOR A Order Code: 1651027
MOLEX - 68784-0001 - CABLE ASSEMBLY, USB A TO MICRO USB B, 1M Order Code: 1617585

Try and work on an antistatic surface if possible. Remove R26. Fit the 6.8pF capacitors into the C16 and C27 footprints. Fit the crystal into the X3 footprint. See the picture below:

Fitted and removed components

Scratch a little patch of the solder resist away from the ground plane on the board so that you can see the copper and tin it. Tin and then solder the negative terminal of the SMD battery holder, through the holder, onto the board. See the picture below.

Battery holder

Don't fit the battery yet, even though it is in the picture. Solder the Kynar wire to the positive terminal of the battery holder. See the picture below.

Positive battery holder terminal

Solder the older end of the wire to the pad in the footprint of R26 indicated in the picture above. You should have something looking like the picture below.

STM32F4DISCOVERY with RTC

Software

I did this on Windows XP SP3 but this could be done on the RPi itself including the downloading to the board as openOCD now supports the STM32F4DISCOVERY onboard programming hardware.

Download and install the Yagarto tools and toolchain. When you install them, make sure there are no spaces in the path e.g. C:\Yagarto.

You can get the latest development version of ChibiOS here. Unpack it somewhere but it might be worth avoiding spaces in the path again..

Download this and unpack it in the "boards" folder. It is modified to show the presence of the 32768Hz crystal. Compare it with the normal ST_STM32F4_DISCOVERY folder using WinMerge to see the differences.

boards folder

Download this and unpack it in the "demos" folder. I have taken and modified the ARMCM4-STM32F407-DISCOVERY-MEMS demo which appears to be a serial port when you plug the board into a PC or RPi. I have added a little code taken from the chibios\testhal\STM32F4xx\RTC folder which is able to set and read the real time clock. WinMerge will be able to show you the differences.

demos folder

Fire up a command prompt and navigate your way in to the ARMCM4-STM32F407-DISCOVERY-RPI folder. Type "make" and it should compile just fine. The command prompt should look something like this:

command prompt

You'll need the "ch.hex" file which was created in the "build" subfolder. You'll need this to program the hex file into the board. You probably ought to upgrade the firmware using this and you'll need the driver so that Windows recognises the board for programming purposes. The mini USB connector is for power and programming. The micro USB connector is directly connected to the micro under software control.

Having programmed the board, the RPi will automatically recognise the micro USB connector as a serial port ( /dev/ttyACM0 ). You can access this, as root, with minicom (type "date get" and "date set") or try a bit of incompetent Python having done an "apt-get install python-serial":
import serial
rpi=serial.Serial("/dev/ttyACM0",timeout=1)
rpi.open()
rpi.write(\rdate get\r")
line1=rpi.readline()
line2=rpi.readline()
line3=rpi.readline()
line4=rpi.readline()
rpi.close()
print line1,line2,line3,line4
 I'm afraid I haven't integrated this into the boot scripts yet.

root python

If you want to test this on Windows, you'll need this driver so that Windows recognises the board as a serial port. When prompted for the driver files they are to be found in "C:\Program Files\STMicroelectronics\Software\Virtual COM Port Driver". You can then use Hyperterminal to type the "date get" and "date set" commands.

hyperterminal

Other commands can simply be added to access the other peripherals using the drivers and example code within the ChibiOS distribution.

Good luck!!