RS485 Problems with USART on STM32F756xxx

[rodi66]

We have to use 4 RS485 interfaces. The first RS485 interface mapped to UART 5 works fine with DE enabled. But we saw, that the same method of configuration for USART 1 and USART2  does not work properly.
The DE signals will be continously high. As so, the receive will not work anyway.

All transmitts over the serial interfaces are implemented via DMA. This works fine!

Pin configuration for UART 5: (working fine)

PC12 (TX), AF 7

PD2 (RX), AF 8 

PC8 (DE), AF7 (UART5 RTS) 

Pin configuration for USART 2: (not working)

PA2 (TX), AF 7

PA3 (RX), AF 7 

PA1 (DE), AF7 (USART2 RTS) 

Pin configuration for USART 1: (not working)

PA9 (TX), AF 7

PA10 (RX), AF 7 

PA12 (DE), AF7 (USART1 RTS) 

As described in reference manual, all features we use are supported:

Table 192. STM32F75xxx and STM32F74xxx USART features; page 1026, in reference manual RM0385 Rev 8

[Pavel A.]

Please imagine that someone else comes to you with this question.

* Do you have enough information to answer?

* What would you advise to them or ask?

[TDK]

Set DE pin as GPIO output and toggle, verify functionality/connectivity on a scope or logic analyzer.

If that works, double check pin and AF mapping, or show relevant code and register values when it's "not working".

[rodi66]

Thanks for your hints.

Maybe there's a link to the STM32F74xxx STM32F75xxx Errata sheet (es0290.pdf)

There is a notice to USART listet: nRTS si active whiel RE or UE = 0.

We will try the described workaround and to toggle the DE pin as GPIO.