SPI CS pin input vs output inactive power consumption

[Miguel Ferreira]

Hi,

I was checking the HAL for SPI in master mode and notice that when pm_device_action_run(<spi_device>, PM_DEVICE_ACTION_SUSPEND); the CS pin isnt changed since it status change after every transmission to output inactive : https://github.com/nrfconnect/sdk-zephyr/blob/v3.5.99-ncs1/drivers/pinctrl/pinctrl_nrf.c#L136

On the other hand, in the spim driver the SS pin is set as input when the pm action is to suspend: https://github.com/nrfconnect/sdk-hal_nordic/blob/8f013ea950f41bf69b18bf688bfb0dd80a3fdc44/nrfx/drivers/src/nrfx_spim.c#L465 . However, the code never reaches this point because the pinctrl is taking care of setting the pins to suspend state.

My question is what is the difference between both, how setting the pin as input differ as setting the pin as ouput inactive.

Thank you!

[Miguel Ferreira]

Hi,

Yes Im cutting the power from the SPI slave IC and after that putting the SPIM in suspend.

[Vidar Berg]

I see. This means that when in the suspended state, the SPI slave IC will be powered by your SS output, hence the additional 7mA idle current. The driver assumes that the IC will remain powered in the suspended state. Since this is not the case for you, you will need to patch the driver to disconnect the SS pin.

[Miguel Ferreira]

Do you know why this is happening ? What Im currently doing is basically setting as input after suspending the SPIM and it does the job.

[Vidar Berg]

I don't know which SPI slave IC you are using, but the problem is likely that you are applying voltage on the SS input when the IC is powered off, see https://www.reddit.com/r/ElectricalEngineering/comments/9fpq4i/what_does_backpowering_a_microprocessor_mean/. 

I will create a ticket in our internal bug tracker and request that we make the driver configure the SS pin as 'input,disconnect' on uninit.

[Miguel Ferreira]

Hmm I see, thanks for pointing out!
But will be a good practice on the uninit to set the SS pin as input for all situations ?
Thanks again!