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From the schematics it seems the MXL7704 is connected to I2C0. So I tried to enable the I2C0 by putting this on /boot/config.txt

dtparam=i2c0=on dtparam=i2c1=on

Unfortunately after a reboot I don't get any device on this bus:

root@raspberrypi:/home/pi# i2cdetect -y 0 0 1 2 3 4 5 6 7 8 9 a b c d e f 00: -- -- -- -- -- -- -- -- -- -- -- -- -- 10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 70: -- -- -- -- -- -- -- --

What's wrong?

Perhaps the INT_SDA and INT_SCL are not connected to any I2C?

What I would like to achieve is to read the 3.3V which is not possible through vcgencmd measure_volts.

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2 Answers 2

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INT_SCL and INT_SDA are not shown as connected to the expansion header (as far as I can see).

Perhaps they are connected to one of the other I2C buses on the Pi4.

I suppose you could try writing drivers for those other buses.

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AFAIK the MXL7704 device on the Pi4 is managed by the Video Core and not accessible to the CPU.

It doesn't "measure" the voltage it uses an in-chip comparator.

The MXL7704 data sheet is readily available.

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  • Are the BCM and the VideoCore two different entities? Commented Jul 12, 2021 at 18:47
  • The Pi SOC has 3 "components" the CPU, GPIO and Video Core which are relatively independent. Commented Jul 13, 2021 at 0:06

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