Posted on October 15, 2012 at 10:53I attempted to use PA9 (through the expansion header) on the STM3240G-EVAL board (as USART1_TX, but it does not matter), to find out it is basically unusable (without removing several parts from the board).
While the text of the documentation suggests that it is enough to remove cable from the FS USB connector (as PA9 is used to monitor the VBUS_FS voltage), this is not sufficient. The documentation also suggests to remove R18, but that's not sufficient either.
A deeper look into the schematics revealed, that there is more circuitry involved, the most problematic of which is a 4.7uF cap. This effectively kills the PA9 if used as output of non-DC-ish signals. It would be trivial to put a zero ohm jumper onto PA9 just after this circuitry, but there is nothing like that there...
I don't need to get this commented; again I am posting in hope this could save anybody else trouble with this issue.
Oh, and of course I also hope that the ST guys read these and would fix this in future revisions of the board, or at least put a note in future revisions of the documentation... if any...
JW
PS. Oh, originally I though about using USART3, which is routed through an RS232 level converter, to find out that the used pins (PC10 and PC11) are shared by SPI3 signals. As SPI is a scarce resource on the STM32F4xx (contrary to USARTs), and as the other SPI3 alternative would kill the JTAG pins, I had to go for USART1 for bootloading. I don't quite understand why couldn't USART1 be routed to the RS232 level converter, too, through jumpers, enhancing substantially the utility of the board.
