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We're developing a compute module board, and added the undervoltage circuit.

Unfortunately, I cannot find where to PWR_LOW_N is actually connected. I have found some saying it's connected to GPIO 35 and GPIO 135 (which seemed to be a typo to me).

Where should we connect the undervoltage detection so the kernel can work directly with it? Or is it something that isn't needed?

Thanks and with best regards, Tom

Tomas Svitil
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  • On the Pi3 a number of devices are connected to a GPIO extender (PWR_LOW_N is accessible via software on address 135). I am unsure about the CM, but presume the same. Your question is unclear - what do you mean by "Where should we connect the undervoltage detection" - the kernel already has support. – Milliways Mar 19 '18 at 09:02
  • Does this mean the BCM does it's own undervoltage detection, or relies on external circuitry? E.g. https://www.raspberrypi.org/documentation/hardware/raspberrypi/schematics/Raspberry-Pi-2B-V1.2-Schematics.pdf On the middle right, there is the undervoltage detector APX803 which is connected to the net PWR_LOW_N. If this net isn't connected anywhere, all it does is blink an led, but I'd think it provides the detection for the BCM. Is PWR_LOW_N net connected directly to the BCM? If so, to which pin? – Tomas Svitil Mar 20 '18 at 09:32
  • To which pin on CM should the undervoltage detection net PWR_LOW_N be connected, so that the kernel can use it. – Tomas Svitil Mar 20 '18 at 09:33
  • I am unsure if this is a language problem but there is no "should" involved. As I stated this is connected to a GPIO extender, NOT to the SOC. See Raspberry Pi Power Limitations – Milliways Mar 20 '18 at 10:00
  • To confirm, a gpio extender is another chip which communicates via SPI/I2C and creates virtual gpio ports? I can't seem to find anything about it online, other than your post on the power limitations.

    I do not understand what happens with the undervoltage signal. I understand that on the Raspi 3, it is connected to the extender and all is OK. On the CM board, is there an undervoltage detection circuit implemented? If not, should it be implemented on the parent board into which the CM is inserted?

     – Tomas Svitil Mar 27 '18 at 11:52

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What did you decide to do, Tomas?

I believe that the schematic of the Under Voltage Detect function was provided by RPi (here: https://www.raspberrypi.org/documentation/hardware/computemodule/schematics.md) simply to show how the detection was done on RPi modules. I suppose the idea of putting the circuit with the CM schematics was that you could do something similar with a CM... so your question about where you should connect it is reasonable given that PWR_LOW_N isn't available on the SODIMM. The function doesn't appear to be on the CMIO development board (though with reduced schematics nothing's certain) and isn't mentioned in the datasheet (https://www.raspberrypi.org/documentation/hardware/computemodule/RPI-CM-DATASHEET-V1_0.pdf) either. Presumably you could use the circuit (or a multi-rail monitor device eg LTC132 if your application is multi-rail), and connect to a GPIO pin.

Clivington
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