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Recommended Pin Usage

A.P

Hi!
We're developing on an NRF52840 and are trying to understand what the meaning of the specified 'recommended usage' is, as its specified in the aQFNTm73 ball assignments
Reading this post dealing with Pin Usage on NRF52840, it seems that any GPIO pin (minus the specified exceptions) can be used for any function available to the chip.
Please note that we have read and understood the recommendation against using the GPIOs near the RF with 'high frequencies' signals, and will adhere to it when allocating functions to GPIOs. We are also aware of specific pins being able to be used as analog inputs and others which can not be used as analog inputs.

Here are our questions:
1. Why are specific pins recommended for use of specific functions if all of them can be used for any function. What makes a certain pin 'recommended' for SPI CLK for example?
2. What are the drawbacks of using non-recommended pins for any of the available functions? Could you specify a list of GPIO to function pairs which will be 'worse', or 'better' than others in some way? ( analog input and high frequency GPIO pins near the RF can be ignored as we understand these points )
3. Which functions are limited to a specific pin? Are the SWD or Trace lines limited to specific pins?
4. Specifically, if you could address the RESET functionality which is recommended to be used in pin P0.18. Will there be any difference in power consumption or the ability to 'wake' the device up from any sleep mode if it is used in a different pin?

Thanks!

  • +1

    Hi Asaf

    1. The recommendations for pins are arbitrary in regards to performance, they are just recommended because these will be the pins that are used by default in our SDK examples, so you'd have to configure say SPI CLK to the pin you want it to be if you're not using the recommended pin. Really, it's more of a suggestion than a recommendation.

    2. No drawbacks, as all GPIOs are the same except the analog input and low drive/low-frequency pins that have some drawbacks or additional features.

    3. The functions limited to specific pins are:

    • The NFC pins (only P0.09 and P0.010 can be used for the NFC antenna, although these can be configured as GPIOs as well if need be).
    • Debug pins (SWDIO, SWDCLK), TRACE pins and the RESET pin are specific to their use-cases, and other pins can not do these operations.
    • Pins XL1 and XL2 (P0.00 and P0.01) are the only pins that can be used as a connection for 32.768 kHz crystal (These can also be configured as GPIOs if need be).
    • Any other pin not described a General purpose I/O in the Pin assignments can not be used as GPIOs.

    4. I'm not entirely sure what you mean here, but you can't use any other pin as a RESET pin the same way as P0.18. The other GPIOs can put the device to deep sleep (System OFF mode) which causes a reset upon wake-up, but won't be able to reset the chip if it's stuck in the firmware.

    I hope these were the answers you were looking for, and if anything is unclear, don't hesitate to ask!

    Best regards,

    Simon

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