How to control multiple Slave select pins using the same MISO, MOSI and SCLK pins in nRF52840?

[Mathangopal]

Hi. I am using three different device which can communicate with the nRF52840 through SPI.

I am using nRF52840 as master and the three device as slave. All the device will be using the same MISO, MOSI and SCLK but different slave select.

How do I configure different slave select in nRF52840? Currently, I am using the SPI example but no luck figuring out.

I will appreciate the help.

Thank you.

[Jared]

Hi,

SPI driver has the option of controlling the CS pin externally. Just set 

    spi_config.ss_pin   = NRF_DRV_SPI_PIN_NOT_USED 

Next, You would have to assign 3 different GPIOs that you use as a CS pin for each slave. You can then toggle the GPIO pin before you transmit/receive.

 

regards
Jared 

[Mathangopal]

Hi. Thanks for the reply. I want my SCLK pins to be separated cos I want the slaves to have different frequencies.

So I am currently having only MISO and MOSI sharing the same pins. Do I require 3 separate SPI instances?

How do I configure this?

Thanks for the help really appreciate it.

[Jared]

Yes, if you want the SCLK to be separate then use 3 instances of the SPIM. Note that this would require that you use different pins for MOSI and MISO as only one peripheral can drive a pin at a time. You could of course connect them externally together. You could also initialize, and then de initialize between each time you change SPI instance if you want to only use MISO and MOSI. But it has it disadvantages. 

Also note that SPIM3 has an errata. 

[Mathangopal]

Hi. Thanks for the reply.

There is a problem when using two SPI instances. Works fine when initialize and de initialize.

Furthermore, it requires to check if the SPI instance is initialized or uninitialized otherwise the program will assert.

Thanks for the help.