RISC OS Open: Forum: Changing GPIO mode on a Pi4

Dec 1, 2020 6:48pm

The SWI

GPIO_WriteMode

not only allows you to set the pull-up/down resistors, event enable etc., but also allows you to change the mode of a particular pin to any of the alt modes for that GPIO. eg. GPIO 12/13/18/19 can have analgoue audio (of a sort) redirected to them by changing to alt mode 4 or 2 depending on which GPIO. I’ve successfully done this on a Pi Zero, Pi 2B+ and 3B+ in RISC OS, and on a Pi4 from RaspberyPi OS. I can’t, however, do it on a Pi4 from RISC OS as whilst the SWIs appear to work, and reading back the mode tells me that it has been changed, nothing actually appears to happen. Does this need to be done differently on a Pi4 (if so, how?), or is there a bug in the GPIO module meaning changing modes just doesn’t work on a Pi4?

Dec 5, 2020 11:21am

Andrew Conroy (370) 740 posts

Does the silence here mean no-one else has tried it, or no-one can replicate the issue? If anyone wants to borrow my PWM Audio board to help find and hopefully fix the issue, let me know.

Dec 5, 2020 11:27am

Julie Stamp (8365) 474 posts

I don’t have a Pi 4.

Does any mode changing work? E.g. can you change between GPIO and serial port?

What mode are you trying to change to (you mention analogue audio – but then PWM?), and what should be happening if the mode change were successful?

Dec 5, 2020 11:42am

Andrew Conroy (370) 740 posts

I’ve only tried changing GPIO 12/13 from alt 0 to alt 4, and GPIO 18/19 from alt 0 to alt 2, which redirects PWM audio to those pins. I haven’t tried doing anything serial related as I’m not sure I’d know how to test that.
If I’m successful, I get PWM audio out via two GPIO pins as well as via the headphone socket (the Pi Zero doesn’t have a headphone socket, hence the original need for my board). I can do this happily on versions pre Pi 4, and I can do it under Raspberry Pi OS on the Pi4, so it is doable, just not on RISC OS at present, it seems. (The “analogue” audio available from the Pi’s headphone socket is actually generated using PWM)

Dec 5, 2020 12:43pm

Andrew Conroy (370) 740 posts

Ah, just remembered, the alt modes I’m referring to are the modes you set them to, which don’t actually correspond to the actual alt mode used by the chip:
2 = Alt 5
3 = Alt 4
4 = Alt 0
5 = Alt 1
6 = Alt 2
7 = Alt 3

Not that that makes any diference to my problem, but might help clear up any confusion(!)

Dec 7, 2020 6:00pm

Timo Hartong (2813) 204 posts

Hi also the gpio command on the Pi 400 has its problems so I’m not amazed that RISC-OS is a bit behind ;-)

Changing GPIO mode on a Pi4

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Dec 1, 2020 6:48pm Andrew Conroy (370) 740 posts	The SWI GPIO_WriteMode not only allows you to set the pull-up/down resistors, event enable etc., but also allows you to change the mode of a particular pin to any of the alt modes for that GPIO. eg. GPIO 12/13/18/19 can have analgoue audio (of a sort) redirected to them by changing to alt mode 4 or 2 depending on which GPIO. I’ve successfully done this on a Pi Zero, Pi 2B+ and 3B+ in RISC OS, and on a Pi4 from RaspberyPi OS. I can’t, however, do it on a Pi4 from RISC OS as whilst the SWIs appear to work, and reading back the mode tells me that it has been changed, nothing actually appears to happen. Does this need to be done differently on a Pi4 (if so, how?), or is there a bug in the GPIO module meaning changing modes just doesn’t work on a Pi4?

Dec 5, 2020 11:21am Andrew Conroy (370) 740 posts	Does the silence here mean no-one else has tried it, or no-one can replicate the issue? If anyone wants to borrow my PWM Audio board to help find and hopefully fix the issue, let me know.

Dec 5, 2020 11:27am Julie Stamp (8365) 474 posts	I don’t have a Pi 4. Does any mode changing work? E.g. can you change between GPIO and serial port? What mode are you trying to change to (you mention analogue audio – but then PWM?), and what should be happening if the mode change were successful?

Dec 5, 2020 11:42am Andrew Conroy (370) 740 posts	I’ve only tried changing GPIO 12/13 from alt 0 to alt 4, and GPIO 18/19 from alt 0 to alt 2, which redirects PWM audio to those pins. I haven’t tried doing anything serial related as I’m not sure I’d know how to test that. If I’m successful, I get PWM audio out via two GPIO pins as well as via the headphone socket (the Pi Zero doesn’t have a headphone socket, hence the original need for my board). I can do this happily on versions pre Pi 4, and I can do it under Raspberry Pi OS on the Pi4, so it is doable, just not on RISC OS at present, it seems. (The “analogue” audio available from the Pi’s headphone socket is actually generated using PWM)

Dec 5, 2020 12:43pm Andrew Conroy (370) 740 posts	Ah, just remembered, the alt modes I’m referring to are the modes you set them to, which don’t actually correspond to the actual alt mode used by the chip: 2 = Alt 5 3 = Alt 4 4 = Alt 0 5 = Alt 1 6 = Alt 2 7 = Alt 3 Not that that makes any diference to my problem, but might help clear up any confusion(!)

Dec 7, 2020 6:00pm Timo Hartong (2813) 204 posts	Hi also the gpio command on the Pi 400 has its problems so I’m not amazed that RISC-OS is a bit behind ;-)