Trying to understand how SPI Slave should be implemented

RE: Trying to understand how SPI Slave should be implemented

SergeyK — Fri, 15 Nov 2019 05:47:01 GMT

Hello! I borrowed a Logic Analyzer, and tried looking at the transactions. I noticed that any errors were also read by LA, so I decided that it's not the chip's fault, but I also noticed that timings were kinda weird and not matching whatever I set script to. So I went back to other SPI library, set a speed of 500khz (which was then properly measurable at LA), and it went with no errors.

I am still not sure that I understand how exactly SPI transaction works on this level (one that I can see trough LA), _besides_ the obvious data transfer on clock edge, like what 'clocked-out' means in nRF52 documentation and what is the purpose of DEF and ORC from my perspective, and what is this small spike on MISO right after CS edge... (I still need to read more articles on SPI).

But I think that this case is solved - I now know how to make a data transfer from RPi and how to receive it properly on nRF52, and maybe even troubleshoot it a bit with logic analyzer

Thank you again!

For future generations: here is my SPI code now

import spidev

spi = spidev.SpiDev()

spi.open(0, 0)

spi.max_speed_hz = 500000
spi.mode = 0

recv = spi.xfer2(bytearray("hello, world", "utf-8"))
print("Recieved %s" % bytearray(recv))

spi.close()

RE: Trying to understand how SPI Slave should be implemented

SergeyK — Wed, 06 Nov 2019 13:33:23 GMT

Okay, thanks a lot! I think I have some SPI theory to read. I also have few other micro controller boards lying around, will try using them; plus I'll check it with logic analyzer when I'll get or borrow one.

RE: Trying to understand how SPI Slave should be implemented

Håkon Alseth — Wed, 06 Nov 2019 13:25:02 GMT

[quote user="SergeyK"]I would really like to hear if my understanding of SPI Slave is correct or if I'm missing something; And what could theoretically be the cause for transfer issues at lower speed.[/quote]

Your understanding is correct. If you want to update the buffer, you call nrfx_spis_buffers_set(), and this is safe to call when the former SPI transaction is done (ie: in the _DONE event of the evt-handler). There isn't anything that should cause it to misbehave at 500 kHz vs. 1 MHz. If you get an ignored transaction (not setting buffer for instance), it should instead clock out the current content of "DEF" register:

https://infocenter.nordicsemi.com/topic/ps_nrf52810/spis.html?cp=3_4_0_5_19_4_34#register.DEF

It should not clock out a partial buffer in that case. That sounds more of a issue on the master side, unless the spi slave buffer is written "mid transfer" by the application firmware, which it does not look like is happening on your side, especially if the test program is that small.

Kind regards,

Håkon

RE: Trying to understand how SPI Slave should be implemented

SergeyK — Wed, 06 Nov 2019 12:50:45 GMT

Hello!

I'm planning on getting a logical analyzer, but this won't happen tomorrow, unfortunately.

As I said, when I had a speed of 5 * 10^4 (I checked now on 5 * 10^5 and still had same issues), I only received successful read once; afterwards I was either missing some bytes, read some bytes wrong, or both. However, when I set the speed to 10^6, it caused data transmission to be a bit more reliable. I tried sending bytes few dozen times at a different times (after some time of nRF52 running, right after reset, etc.), and I still do receive garbage, but every third time.

I would really like to hear if my understanding of SPI Slave is correct or if I'm missing something; And what could theoretically be the cause for transfer issues at lower speed.

Thanks.

RE: Trying to understand how SPI Slave should be implemented

Håkon Alseth — Tue, 05 Nov 2019 07:26:10 GMT

Hi,

Do you have a logic analyzer so that you can look at the SPI signals on the bus itself?

How often do you read out with the unwanted characters ? Once in a while, or every time?

Kind regards,

Håkon

RE: Trying to understand how SPI Slave should be implemented

SergeyK — Thu, 31 Oct 2019 20:06:02 GMT

I have noticed that setting higher speed (50000 -> 1000000) causes transmissions to be more reliable and to occur in a single event more often, which is kinda counter-intuitive for me, but I hope this gives someone a clue to what is going on here.