Baud rate for SPI

RE: Baud rate for SPI

Riccardo78 — Thu, 19 Mar 2020 15:59:52 GMT

Hi Jev,

thanks for the feedback.

RE: Baud rate for SPI

jev — Thu, 19 Mar 2020 15:57:54 GMT

Well, I ended up using SPIM at almost 9600 bps, coding 1 bit in a SPI byte transfer. The spec says +/- 10% timing is allowed and I'm 1.5% fast so no worries there. Needed to add an IDLE bit before the start of the transfer because SPI starts clocking rather late. Prolly some optimizations still possible.

Receiving is quite a bit more complicated since that is asynchronous. For now, it seems to work.

The one thing it won't do is collision detection. O well...

RE: Baud rate for SPI

Riccardo78 — Sun, 08 Mar 2020 16:41:12 GMT

Hi jev,

at the end it was possible to set lower speed than the ones reported in the manual (regarding the SPI peripheral), but note that there is a kind of approximation. I do not remember exactly the value of that (might be rounded_value = (value + 0x800) & 0xFFFFF000), so the consequence was that in my case i could not reach the SPI baudrate i wanted (9600 bit/sec).

So, as you said, I tried the SPI slave feeding the clock with another pin of the Nordic configured as GPIO controlled with a TIMER (PPI approach). In order to control the Chip Select of the SPI I used to activate the its pull-down.

I did a couple of test and it seems to work properly. In our product we did not adopt the SPI slave then, so I did not perform further and thorough tests. But I hope this could help you to give a try.

Let me know if it works :)

Riccardo

RE: Baud rate for SPI

jev — Sun, 08 Mar 2020 15:22:19 GMT

Did you get this low speed clocking issue solved?

I am trying to solve the same problem; was thinking to use SPIS instead of SPIM. SCK could be fed from a timer (possibly externally connected), which is costing me 2 extra pins. Don't know how that'll work for input though since we would need the clock to start from GPIO or continuously monitor if there's any data coming in from SPI.

If that won't work I'll just have to go with a PPI based GPIO / timer solution where a pin change triggers a timer value to be written into memory. Bit extra software required, should work for input but not for output. I don't think it's possible to drive GPIO from DMA, so I needed to come up with another clever solution there.

RE: Baud rate for SPI

Riccardo78 — Fri, 18 Oct 2019 10:53:32 GMT

Hi,

ok, thanks again for the support.

riccardo

RE: Baud rate for SPI

Dmitry — Fri, 18 Oct 2019 08:48:08 GMT

Hi,

SPI and TWI share address space and some registers, but they're different peripherals. I don't know the internals, one can suppose that they're clocked from different PCLK sources.

RE: Baud rate for SPI

Riccardo78 — Fri, 18 Oct 2019 08:20:14 GMT

Hi Dmitry,

Once more thing.

I noticed that the Frequency register of the SPI is shared with TWI (I2C) one. But the values reported in the Product Specification v1.4 (see the image below) are different from the ones I obtained from the formula in the post. Are they computed with a different formula or am I doing something wrong ?

Values obtained through the formula in the post

Could you kindly give your feedback on those values please ?

Thanks in advance !

Riccardo

RE: Baud rate for SPI

Riccardo78 — Thu, 17 Oct 2019 21:21:01 GMT

Hi Dmitry,

Ok. Actually my goal it would be to use the spi peripheral at 2400 bps to implement a driver for the DALI protocol. So, if there are no official restrictions about lower baudarate, I might give a try.

Thanks again.

Kind Regards

Riccardo

RE: Baud rate for SPI

Dmitry — Thu, 17 Oct 2019 21:03:18 GMT

I didn't find it in official documents.. just can say that I used it to turn SPI interface into a simple tx-only UART, and it worked at 115200.

RE: Baud rate for SPI

Riccardo78 — Thu, 17 Oct 2019 20:53:12 GMT

Hi Dmitry,

Thansk so much for the prompt reply.

Is the formula described in the post also available in some official documents ?

Is there any problem/issue if we use a peripheral with a lower baudarate ?

Thanks again!

RE: Baud rate for SPI

Dmitry — Thu, 17 Oct 2019 19:28:39 GMT

Hi Riccardo,

you can find the formula in this thread, it's the same for SPI.