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USART In Synchronous Mode: SOME Data Incorrect

Question asked by Joshua on Jan 22, 2015
Latest reply on Jan 23, 2015 by waclawek.jan
Now that I've got my USART working with my DMA to do two 8-bit transfers per 16-bit word, I have encountered another problem: incorrect data. I know the mantra for serial comms when data is wrong is baud rate, baud rate, baud rate, but this one's a bit different: since I'm running in synchronous, my receiver isn't expecting any particular baud rate, so it's not a % error issue. 

I send six bytes (3 16-bit words) via synchronous USART to my ASIC. My ASIC is expecting a SPI-type transfer, so it's happy with any speed up to 30MHz. I'm running the STM32F427 at 168MHz, and dial the baud rate down to something reasonable when I set up the USART. 

The issue is that the SECOND byte (out of the six), and ONLY the second byte, is wrong 90% of the time. I've scoped it, and sometimes it's shifted, sometimes a high is too long, sometimes too short, etc. About 10% of the time it gets it right. 

I can post my code if need be, but since 5/6 bytes transfer fine, I'm not so sure that would help. I have tried dialing the baud rate to 21MHz,  10.5MHz, 5.25MHz, and even tried hard coding the "standard" RS-232 values all the way down to 9600bps. No change either way. 

Does this sound like some sort of reflection issue on the transmission line?