DIGI2 PRO V2.2 + DSP clock circuit

HiFiBerryOS 20230404, Digi2 Pro + DSP, Pi 4 Rev. 1.4 

In the GPIO usage documentation for the Digi2 Pro, it says that GPIOs 5 and 6 are used for the clock circuit and cannot be used for anything else.

GPIO usage of HiFiBerry boards | HiFiBerry

When I use sox to play a test tone, and probing with an oscilloscope, I see the I2S signals on GPIO18, 19, and 21, but do not see any waveform on GPIO5 (pi header pin 29) or GPIO6 (pi header pin 31).

Two questions:

Is this my failure in probing or should I expect no clock signal on these lines?

Does the pi send the optional I2S MCLK signal to the Digi2 Pro and/or the DSP?  (Do these HiFiBerry cards need it?  If so, on what pi header pins?)



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