Using GPIOs marked as "standard drive, low frequency" for SPI on nrf52840

[Akshay]

I want to understand the meaning of "standard drive, low frequency" GPIO pins. The datasheet mentions to use these pins for frequencies below 10 KHz. Does this limitation apply if I use SPIM on these pins? I tried running the SPIM at 8MHz and did not see any problem. Is it recommended to use only high frequency pins for SPIM?

[haakonsh]

The limitation is due to HF interference being injected into the RF radio. Do not use pins described as low frequency only as high frequency IOs (SPI).

[Pink Fluffy Unicorns BV]

Hi Akshay and haakonsh,

From the datasheet, the maximum rise and fall time for the standard drive pins is up to 25 ns @ 50 pF load. (see Product Specification V1.1, paragraph 6.9.3.1 "GPIO Electrical Specification"). This means that these pins should be fine up to at least 10 MHz. It would be nice to have somebody at Nordic confirm or refute this, since a 10 kHz maximum switching speed is insufficient for any modern serial connection. It just seems off.

Cheers,

Casper

[cbd]

I believe that the restriction is due to their proximity to the RF path and therefore causing cross talk and interference with any radio transmission/reception.

I guess that you could go ahead and use them in anyway that you wish, but be prepared for possible failure of your product certification once the RF testing gets underway.

[Pink Fluffy Unicorns BV]

cbd, ah, that sounds reasonable. I also checked the pin diagram of the nRF52840 in aQFN-73 and all the standard drive, low frequency pins are indeed located on the two sides of the IC closest to the RF output.

Surely we can work around that in software, if we'd run into problems. We would have to make sure that no RF transmission is taking place during action on the IO pins.

[Simonr]

Hi

Sorry for the late reply, but Håkon is on vacation, and we've just now gotten to his backlog. I can confirm what Akshay is saying. They are not restricted to 10kHz, just recommended to not be set higher due to interference issues with the radio. You're also correct that you can work around it if the radio is not going to run while you do SPI transmissions, but if possible we suggest using the "regular" GPIOs for high freq. serial communication, etc.

Best regards,

Simon