NRF52 not reading third SPI byte

RE: NRF52 not reading third SPI byte

Nguyen Hoan Hoang — Fri, 18 Jan 2019 21:41:30 GMT

Can use this SPI driver code as reference. Same code works across nRF5x series with supports for both master/slave, IO or DMA.

RE: NRF52 not reading third SPI byte

David Andresky — Fri, 18 Jan 2019 18:25:33 GMT

// Refer to "Figure 58: SPI master transaction" in the nrf51 reference manual
void spi_master_transfer(uint8_t const * p_tx_buf, size_t tx_buf_len, uint8_t * p_rx_buf, size_t rx_buf_len)
{
    VERIFY_TRUE_VOID(spi_master.open);
    VERIFY_TRUE_VOID(BIT_IS_CLEAR(NRF_GPIO->OUT, spi_master.pin_slave_select));

    size_t const max_length = MAX(tx_buf_len, rx_buf_len);

    VERIFY_TRUE_VOID(max_length > 0);

    size_t bytes_sent = 0;
    size_t bytes_received = 0;

    // kick off the process by loading the double buffered TXD
    for (int i=0; i<2; i++)
    {
        if (bytes_sent < max_length)
        {
            if (p_tx_buf != NULL && bytes_sent < tx_buf_len)
            {
                NRF_SPI->TXD = p_tx_buf[bytes_sent];
            }
            else
            {
                // send a dummy byte in order to facilitate reading bytes we care about
                NRF_SPI->TXD = SPI_DEFAULT_TX_BYTE;
            }
            bytes_sent++;
        }
    }

    uint8_t rx_byte;

    while (bytes_received < max_length)
    {
        while (!NRF_SPI->EVENTS_READY) {}   // TODO: driver locks up here with optimization at O3

        rx_byte = NRF_SPI->RXD;

        if (p_rx_buf != NULL && bytes_received < rx_buf_len)
        {
            p_rx_buf[bytes_received] = rx_byte;
        }
        bytes_received++;

        if (bytes_sent < max_length)
        {
            if (p_tx_buf != NULL && bytes_sent < tx_buf_len)
            {
                NRF_SPI->TXD = p_tx_buf[bytes_sent];
            }
            else
            {
                // send a dummy byte in order to facilitate reading bytes we care about
                NRF_SPI->TXD = SPI_DEFAULT_TX_BYTE;
            }
            bytes_sent++;
        }
        
        // Note: Initial implementation for the nRF51 based modules did not require these nop's. However, the nRF52 was failing
        // the self-test and after some investigation I discovered that the third byte in the read_jedec_id command was being
        // transfered by the chip but was not being read into the rx buffer. After reading paragraph 48.1.3, SPI master transaction
        // sequence, of the product specification I attempted the nop's and was able to read in the third byte and pass the test.
        // I first attempted 5 nop's but saw no change. I then arbitrarily tried 15 nop's and saw success. I added 5 more for
        // good measure but the bootloader was not working so I increased to 40 and all issues were resolved.
        if(bytes_received == 2)
        {
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
            __asm("nop");
        }
    }

    NRF_SPI->EVENTS_READY = 0;
}

RE: NRF52 not reading third SPI byte

Susheel Nuguru — Wed, 16 Jan 2019 10:36:35 GMT

Thanks for lettings us know about this. I do not understand why this fixes the issue. They should not have been needed if you are reading them on the condition of getting READY event.

Just for future reference, can you please give us the code snippet to have a little more insights about the driver state at the time you add these nops.

RE: NRF52 not reading third SPI byte

David Andresky — Mon, 14 Jan 2019 15:39:36 GMT

Hi Susheel,

We have our own driver, and like I said, it works on the NRF51.

In section 48.1.3 of the product specification there are a couple paragraphs about the READY event and reading bytes B and C. It is difficult to interpret but on a whim I decided to add 15 nops before reading in the third byte and this fixed the issue. The 15 nops added another half clock cycle to the gap and now I can read the data. I have run tests reading thousands of bytes in 256 byte increments without failure.

RE: NRF52 not reading third SPI byte

Susheel Nuguru — Wed, 09 Jan 2019 09:51:52 GMT

You should still be able to use the SPI module. I am guessing/hoping that you are using latest SDK drivers to communicate to your SPI module on nRF52832. We have not known any other issue apart from the extra clock issue already mentioned in the Errata and this one does not match your symptoms.

Try your communication using SDK15.2 nrfx_spi.c driver