STM32F407-Discovery, VBUS/PA9 and SPI-Slave

mats · ‎2014-04-09

Posted on April 09, 2014 at 16:00

I am using SPI2 as a slave with the help of DMA for fetching a stream with 16byte-burst of data. To test my application i have been looping many GB of data without any bit error so i am confident that my application is stable.

However, when i apply 5 Volts to GPIO PA9 the data i receive gets corrupt at random intervals.

PA9 is configured as digital input (and i tried with analog input, same result).

I have checked with an oscilloscope/logic analyzer and there is nothing wrong with the data sent to the SPI-slave.

If i apply 4.8 Volts to PA9 i can not reproduce the issue. If i apply 4.9 Volts i get about 1% error. At 5.0 Volts 50% of my bursts are corrupt and at 5.1 Volts 99% is corrupt.

I have tried this om three different boards with the same result.

Have anyone seen this issue or know about some way to solve it?

waclawek.jan · ‎2014-04-09

Posted on April 09, 2014 at 16:40

1. There is USB-related circuitry connected to PA9 on the STM32F4DISCOVERY. See the board's schematics. If not anything else, the datasheet of the switch states that VOUT should never be higher than VIN, without specifying what happens, if it is. I'd remove the switch as the first step.

2. Note, that pins configured to analog function are NOT 5V tolerant.

JW