SPI Slave RX data shifted

Konami · ‎2017-11-21

Posted on November 21, 2017 at 17:02

I'm having an issue with the LL SPI driver on the STM32L4 configured as SPI slave. My Tx buffer contains {01, 02, 03, 04, 05, 06, 07... }. When I read 7 bytes repeatedly from the slave using the Aardvark SPI master I get the following responses:

01 02 03 04 05 06 07

08 09 0A 01 02 03 04

05 06 07 01 02 03 04

...

Resetting the master doesn't change anything. Resetting the STM32 slave will start the pattern over. It seems like the STM32 is trying to sent 10 bytes instead of the 7 the master is reading and the last 3 bytes are persisting in the TX buffer. Is this possible? How do I fix this?

#stm32l4-spi

Konami · ‎2017-11-21

Posted on November 21, 2017 at 17:45

Okay, I've discovered that when I call the LL_SPI_Enable function it actually triggers the SPI2_IRQHandler 3 times with the LL_SPI_IsActiveFlag_TXE flag set. This is causing me to to send 3 bytes that weren't actually requested by the master.

Any ideas as to why or if this is expected?

S.Ma · ‎2017-11-21

Posted on November 21, 2017 at 18:17

Make sure that if you use bytes, you read/write the SPI DR as a 8 bit write bus access.

The SPI has I think a 32 bit TX fifo: you can write 4 bytes one shot if configured like this.

All this to avoid dead time between byte transfer when using interrupt (non DMA) based systems.

Konami · ‎2017-11-21

Posted on November 21, 2017 at 19:40

I am using it interrupt mode rather than DMA actually, but I think I'm doing the R/W with the correct functions:

LL_SPI_ReceiveData8()

LL_SPI_TransmitData8()