SPI with multiple chip selects

[nszmnsky]

As I understand the SPI HW driver documentation, it appears to be at least biased for using a single chip select (slave select in the API). I have an application where I have 5 devices on the SPI bus.

Should I create an SPI master configuration structure for each of the 5 devices? Only the SPI_Pin_SS would be different in each of them, and I think I would need to close and open SPI to switch between the devices.

Or can I use a single structure with spi configuration function to configure the SPI pins (CLK, MOSI, MISO) and manually configure the chip selects? And then when I want to change device I want, just change the SPI_Pin_SS?

Another way? After reading RK's answer, began to wonder if could use GPIOTE to handle the multiple chip selects. Would it work to assign SPI_Pin_SS to an "unused" pin, and cofigure GPIOTE task to toggle desired CS when SPI toggles _SS ? I haven't explored GPIOTE at all yet so this may be a naive idea.

[nszmnsky]

I discovered that the chip select can be changed in the spi_master_instance that is created when SPI is opened. I wrote this function to do it:

void spi_master_change_cs(spi_master_hw_instance_t spi_master_hw_instance, uint32_t cs_pin)
{		
  volatile spi_master_instance_t * p_spi_instance = spi_master_get_instance(spi_master_hw_instance);
  p_spi_instance->pin_slave_select = cs_pin;
  //A Slave select must be set as high before setting it as output,
  //because during connect it to the pin it causes glitches.
  nrf_gpio_pin_set(p_spi_instance->pin_slave_select);
  nrf_gpio_cfg_output(p_spi_instance->pin_slave_select);
  nrf_gpio_pin_set(p_spi_instance->pin_slave_select);
}	

and called like this

spi_master_init(SPI_MASTER_0, spi_master_0_event_handler, true);

for (;; )
{
		spi_send_recv(SPI_MASTER_0, m_tx_data_spi, m_rx_data_spi, TX_RX_MSG_LENGTH);
		nrf_delay_ms(1);
		spi_master_change_cs(SPI_MASTER_0,5);
		spi_send_recv(SPI_MASTER_0, m_tx_data_spi, m_rx_data_spi, TX_RX_MSG_LENGTH);
		nrf_delay_ms(1);
		spi_master_change_cs(SPI_MASTER_0,6);
		spi_send_recv(SPI_MASTER_0, m_tx_data_spi, m_rx_data_spi, TX_RX_MSG_LENGTH);
		nrf_delay_ms(10);
		spi_master_change_cs(SPI_MASTER_0,2);
}

Not perfect, still have to wait until SPI finishes previous transmission before changing hence the delays.

[Gabriel C.]

Thanks a lot for this answer. I have 4 FRAM memories on on single SPI bus and I was breaking my head to know why it was not switching the CS. Thanks again!

[nszmnsky]

Gabriel, I was surprised that the original Nordic code didn't deal with multiple cs better. I've just posted another modification of the spi_master code that you might be interested in. SPI mask code