Issues w/ SPI communication using both NRF21540 and IMU slaves simultaneously

RE: Issues w/ SPI communication using both NRF21540 and IMU slaves simultaneously

Machado94 — Thu, 24 Aug 2023 15:22:26 GMT

Hi,
I'm observing the following behaviour with the logic analyzer:

FEM CSn is allways Low except for small periods (maybe when it's not on PDN), image 1
FEM can succesfully write SPI data within those periods, image 2
Since FEM_CSn is pulled low most of the time, when the IMU_CSn is enabled the SPI enabled is immeadiatly pulled back up, since the FEM_CS, image 3
Therefore, only the FEM can communicate via SPI, the IMU is unable to communicate.

Follows attached illustrative images of the described behaviour.

Image 1:

Image 2:

Image 3

Best regards,
Pedro

RE: Issues w/ SPI communication using both NRF21540 and IMU slaves simultaneously

Bendik Heiskel — Tue, 22 Aug 2023 12:05:34 GMT

Hi,

By default the CS pin for the nRF21540 will be pulled low with the PDN pin, this is part of the workaround for errata 13.

The main reason for recommending not sharing the SPI bus is due to spikes on the MISO line from the nRF21540 when it the PDN pin is toggled. It should be possible to share the SPI bus as long as no SPI communication is taking place as the nRF21540 is woken up from sleep.

What kind of issue are you seeing? Can you attach a logic analyzer to the SPI and the nRF21540 GPIO lines?

Best regards,

Bendik