I am on a raspberry pi 4. Using both SPI's with no need for either chip select. I'm running out of I/O and could really use those. Does anyone know if there is a way to repurpose the SPI chip selects as gpio while still using SPI? Thank you for your time!
2 Answers
If you are not using them for SPI you can use them for any of their alternative functions. So yes, just set them to INPUT mode or OUTPUT mode using the GPIO library of your choice.
However the current Linux SPI driver leaves them in OUTPUT mode as it drives the select lines itself. You might have to use a different SPI driver.
If you are using pigpio set the ux bit to 1 for each GPIO you don't want to be used as a SPI chip select.
u2 is only relevant to the auxilary SPI. u0 and u1 are used by both SPI devices.
If you are using the standard SPI driver perhaps select a GPIO not available on the expansion header.
e.g.
dtoverlay=spi0-2cs,cs0_pin=44,cs1_pin=45
- Which leads to the next question... :)Seamus– Seamus2021-02-20 18:55:13 +00:00Commented Feb 20, 2021 at 18:55
- The stars have aligned as I'm already using PIGPIO! I'm not quite understanding the spi flag setup for ux though. Is each ux bit for the associated CE? Where first cs on main spi would be u0 and 2nd would be u1? Then for aux spi its the same but it has three cs's so u2 would be in use as well. Am I understanding that correctly?chrismec– chrismec2021-02-20 19:54:10 +00:00Commented Feb 20, 2021 at 19:54
- Awesome! Thank you very much : )chrismec– chrismec2021-02-20 22:45:25 +00:00Commented Feb 20, 2021 at 22:45
- I'm guessing pigpio uses the standard spi driver when not bit banging. I was not able to use CE0 as a gpio simply by setting the ux bits to 1. Changing the chip selects to IO not available on the header with dtoverlay cleared that up though. Thanks again to all yall.chrismec– chrismec2021-02-21 00:16:25 +00:00Commented Feb 21, 2021 at 0:16
- @chrismec The SPI dtoverlays have no relevance to pigpio. pigpio would take no notice of them. However if you were using another kernel function to assign those GPIO it would see them as in use.joan– joan2021-02-21 08:48:07 +00:00Commented Feb 21, 2021 at 8:48
If you're using RaspiOS 32 with the latest 5.10.11 kernel then you have /boot/overlays/spi0-1cs.dtbo and /boot/overlays/spi1-1cs.dtbo
Those free up the second CS pin. There's even an option to free up the MISO pin if you're doing send-only SPI transfers.
From /boot/overlays/README
Name: spi0-1cs Info: Only use one CS pin for SPI0 Load: dtoverlay=spi0-1cs,<param>=<val> Params: cs0_pin GPIO pin for CS0 (default 8) no_miso Don't claim and use the MISO pin (9), freeing it for other uses. Name: spi1-1cs Info: Enables spi1 with a single chip select (CS) line and associated spidev dev node. The gpio pin number for the CS line and spidev device node creation are configurable. N.B.: spi1 is only accessible on devices with a 40pin header, eg: A+, B+, Zero and PI2 B; as well as the Compute Module. Load: dtoverlay=spi1-1cs,<param>=<val> Params: cs0_pin GPIO pin for CS0 (default 18 - BCM SPI1_CE0). cs0_spidev Set to 'disabled' to stop the creation of a userspace device node /dev/spidev1.0 (default is 'okay' or enabled). 
- ...and as I suggest in my answer you can also select GPIO not on the expansion header.joan– joan2021-02-20 21:42:03 +00:00Commented Feb 20, 2021 at 21:42