SPIS ORC register not being used

[Peter Myerscough-Jackopson]

I have managed to get the SPIS block working using standard sets of read and transmit buffers. This all works.

I am now trying to use the ORC register to send a single byte to be transmitted rather than supplying a separate buffer, because in the main I wish to only transmit one byte and receive many. I don't mind if this byte is repeated either. The byte is a read command to an external SPI device, so doesn't need a long buffer.

If I set the NRF_SPIS1->MAXTX = 0, the ORC value does not appear on the SPI output, AND the status register does not show an over read. I get an all high signal.

If I set the NRF_SPIS1->MAXTX = 1, and set NRF_SPIS1->TXDPTR = &buffer, the first byte of the buffer is transmitted, but the ORC value still does not appear on the SPI output, BUT the status register does show an over read. The SPI transmits an all high signal when I have set the ORC value to something else. I also noticed that the AMOUNTTX is only equal to 1 when I know from my signal analyser that there has been 2 bytes transmitted, and AMOUNTRX = 2 as well.

Is this a known errata? or am I missing something

[Stefan Birnir Sverrisson]

Hi Peter

Are you setting your NRF_SPIS1->MAXTX = 0 in the spi_slave_init function? If so, that does not work because the MAXTX configuration is overwritten when you call spi_slave_buffers_set. To adjust the MAXTX, modify the third parameter when you call spi_slave_buffers_set function. The spi_slave_buffers_set function is called for the first SPI transaction in spi_slave_example_init but for all subsequent SPI transaction in the SPIS handler, i.e. spi_slave_event_handle. So to set MAXTX as e.g. 3, set

err_code = spi_slave_buffers_set(m_tx_buf, m_rx_buf, 3, sizeof(m_rx_buf));
APP_ERROR_CHECK(err_code);

[Peter Myerscough-Jackopson]

Hi Stefan,

I have tried the following line:

spi_slave_buffers_set(m_tx_buf, m_rx_buf, 0, sizeof(m_rx_buf));

This transmits the first byte of the m_tx_buf, followed by all 0s. (I am using a logic analyzer to see the lines.)

Alternatively, after removing the NULL pointer detection, in spi_slave_buffers_set(...), I tried this code:

spi_slave_buffers_set(NULL, m_rx_buf, 0, sizeof(m_rx_buf));

This transmits all 0s, and not the ORC byte. I have also tried setting all four bytes of the ORC register to see if the higher bytes are actually being used.

N.B. If I transmit one byte, i.e. spi_slave_buffers_set(m_tx_buf, m_rx_buf, 1, sizeof(m_rx_buf)) the ORC byte is transmitted.

[Stefan Birnir Sverrisson]

I actually tried to set MAXTX = 1 and MAXTX = 3 and that worked, but after trying MAXTX = 0, I get the same results as you, there is nothing sent out on MISO. According to the reference manual, ORC should be clocked out on MISO if transmitted bytes exceed the value of MAXTX, including when MAXTX = 0. The most knowledgeable people on SPIS are not in office today, I will discuss the issue with them when they are available. Meanwhile I report this as a bug. If the slave has no data to send, I guess the workaround could be to set MAXTX = 1 and write the ORC value manually to the TX buffer.

[Peter Myerscough-Jackopson]

Thank you for looking at this. I will convert your previous comment to an answer, and select it once you confirm with the SPIS people when they get back in...

[Peter Myerscough-Jackopson]

please can you alter this or comment if after chatting with the SPIS people this needs updating.