USART in SPI mode

[EE_CV]

Is it possible to configure the USART as SPI master, with SPI slave having 16 bit data frames (AD5592R) ?

[Tesla DeLorean]

I don't see that working

[EE_CV]

Is that because the USART in SPI mode only support 8-bit data?

[Tesla DeLorean]

Well arguably it can deal with 9 bits and some stop/start configurations, but I don't see a winning move here in terms of generating usable signalling. I'm lacking scope/context.

Why can't you use an SPI peripheral? Which STM32 part are we talking about?

Are you trying to send data, receive, or combination of both? At what speed?

[S.Ma]

If one SPI is used as master and you need another one, you could use alternate GPIOs and dynamically (and time multiplex) its use. Otherwise, was told USART can be a slower speed version of SPI Master with CK output pin. SPI by SW emulation using GPIOs in push-pull mode (if 4 wires configuration) can give you few MHz frequency raw.

[EE_CV]

Yeah we will probably try GPIOs in push-pull mode and see how that works out. Without a timer connected to the USART clock it's not clear to me if there is any advantage configuring the USART peripheral at all. 1MHz raw is fine for us as long as it doesn't tie up the CPU for too long, which remains to be checked. At the very fastest we need to communicate with the SPI slave every 3ms.

[waclawek.jan]

Isn't 16 bit = 2 x 8 bits?

What's the issue? I presume you are talking about the synchronous mode of USART. Is the peripheral in question picky about clocks being continuous?

https://community.st.com/s/feed/0D50X00009bLS7GSAW

JW

[EE_CV]

Yes I am looking at synchronous USART used as SPI master. Would the USART insert a stop and start bit between each 8 bits in that mode? If so wouldn't that mess up the slave?

[waclawek.jan]

> Would the USART insert a stop and start bit between each 8 bits in that mode?

Internally it times the stop/start bit, but won't issue clocks for them, so this demonstrates itself as a gap in the clock, as I wrote in the post I linked above.

> If so wouldn't that mess up the slave?

No, except if the slave requires continuous (strictly regular) clock.

JW