Turris Omnia routers are equipped with GPIO connector. How to use it? We have added detailed instructions.
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Is it useful or useless function?
From the perspective of the Universe is it offcourse useless…
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From my perspective it is useful. It currently connnects to a GPS module with PPS. There are more ideas on my mind but none realized for the moment.
Will there be a description how to use PWM?
I would like to control the fan.
The GPIO on the Marvell SoC does not support real PWM.
To control a 4pin fan you could use a small MCU like ATtiny85, a few resistors and an optional transistor. The connection to the Omnia could be I2C, UART or plain GPIO. Plain GPIO would not support speed readback and a maximum of 16 speed steps using 4 GPIO. The I2C or UART connections can make the fan speed available.
OK, but will the software PWM ?
Like this: PWM emulation with GPIO [Old OpenWrt Wiki]
100 Hz is far too slow to get a fan controlled. Target about 25000 Hz.
But there may be another option: the GPIO parts may have a blink function. As it seems to be unused you may get 2 PWM controllable pins. GPIO 18 and any other GPIO. The GPIO chips seem to be feed by the 25 MHz clock so it is realistic to get fast enough for a fan. It would only need a DTB change as it is already in the kernel and used for the WRT1900AC.
Now the bad news: only Armada 370 and XP seem to support this. I doubt this but who knows. One way to check would be writing the correct values to the registers and try.
Patches 202 to 206 at https://github.com/CZ-NIC/turris-os/tree/test/target/linux/mvebu/patches-4.4 enable this for the WRT1900AC and are included in the Turris Omnia kernel.