Problems to manage SPI pins to communicate with the slave.

[nemunoz]

Hi to everyone!

I'm trying to use the nRF52832 board as master to use an external device (as slave) through SPI. I take the example peripheral/SPI to start to learn how to work with this. In the example I see the event but I don't found when is triggered. link

And, to comunnicate with the slave, the master has to accomplish with one condition: When the master pulls low the ss pin, it also has to wait until MISO pin goes low before starting to transfer. But I don't know how to manage the pins and indicate the way to comunicate with the slave device.

Any idea?

Thank in advance.

[Jørgen Holmefjord]

I'm sorry but I don't understand your questions. What event are you talking about? Where does the requirement that MISO needs to be driven low come from? If you are using the SPI driver, this will handle the pins for you.

[nemunoz]

The event handler is the third parameter of nrf_drv_spi_init function (the event type is nrf_drv_spi_evt_handler_t)

The slave device is a radio frecuency chip CC1125 and the user's guide, in the page 8 first paragraph, says:

"When CSn is pulled low, the MCU must wait until CC112X SO pin goes low before starting to transfer the header byte. This indicates that the crystal is stable. Unless the chip was just reset or was in SLEEP or XOFF state, or the XOSC configuration has been altered, the SO pin will always go low immediately after pulling CSn low."

[Jørgen Holmefjord]

The NRF_DRV_SPI_EVENT_DONE event is passed to the event handler when the transfer is doen. The Slave Select pin is optional in the SPI master driver config struct. If you set this pin to NRF_SPI_PIN_NOT_CONNECTED, you can control the SS/CS pin yourself. You can then check if the MISO pin goes low before starting the SPI transfer. Something like the below code should work:

void spi_event_handler(nrf_drv_spi_evt_t const * p_event, void * p_context)
{
	nrf_gpio_pin_clear(SPI_SS_PIN);
}

void spi_transfer()
{
	nrf_gpio_pin_set(SPI_SS_PIN);
	while (nrf_gpio_pin_read(SPI_MISO_PIN))
	{
	}
	APP_ERROR_CHECK(nrf_drv_spi_transfer(&spi, m_tx_buf, m_length, m_rx_buf, m_length));
}

void main()
{
	nrf_gpio_cfg_output(SPI_SS_PIN);
	
	nrf_drv_spi_config_t spi_config = NRF_DRV_SPI_DEFAULT_CONFIG;
    spi_config.ss_pin   = NRF_SPI_PIN_NOT_CONNECTED;
    spi_config.miso_pin = SPI_MISO_PIN;
    spi_config.mosi_pin = SPI_MOSI_PIN;
    spi_config.sck_pin  = SPI_SCK_PIN;
	
    APP_ERROR_CHECK(nrf_drv_spi_init(&spi, &spi_config, spi_event_handler, NULL));
	
	spi_transfer();
	
	while(1)
	{
		
	}
}

[nemunoz]

yeah! It works! Thank you so much! I didn't know I could do that!

I'm sorry for answer you so late but I was also having problems to use the slave through SPI (an independent problem) so couldn't try your solution before.

[nemunoz]

Sorry but I find a mistake in the code. The clear and set functions are in the wrong positions. To fix it you must interchange them.