Audio capture on nRF5340-DK with external I2S ADC (PCM1808) - MCK signal quality and VREF stuck at 0V

RE: Audio capture on nRF5340-DK with external I2S ADC (PCM1808) - MCK signal quality and VREF stuck at 0V

Kenneth — Wed, 29 Apr 2026 15:34:45 GMT

It's not possible to get much faster than 16KHz no, I guess the fastest would be 1.333MHz/64 = 20.83kHz.

https://docs.nordicsemi.com/bundle/ps_nrf5340/page/pdm.html#ariaid-title24
https://docs.nordicsemi.com/bundle/ps_nrf5340/page/pdm.html#ariaid-title28

Kenneth

RE: Audio capture on nRF5340-DK with external I2S ADC (PCM1808) - MCK signal quality and VREF stuck at 0V

Turbo J — Wed, 29 Apr 2026 15:06:55 GMT

S0S1 increases signal rise time too much for most use cases. I believe you can't use this past 1-2 MHz just due to pin capacitance alone. Faster outputs require H0H1 or E0E1 (on supported pins).

Ringing is often an oscilloscope artifact.

Quick testing at 32kHz stereo produced audible buffer underrun. No idea why, might indeed be a hardware limit.

RE: Audio capture on nRF5340-DK with external I2S ADC (PCM1808) - MCK signal quality and VREF stuck at 0V

inescoding — Wed, 29 Apr 2026 14:35:27 GMT

Thanks for the input. For the MCK drive mode, I do have H0H1 set in pinctrl for the MCK pin.
I recently switched to S0S1 to reduce ringing, which helped with undershoot
but the signal is still sinusoidal. Is there something beyond drive mode
that would affect the MCK output shape at 8 MHz?
On the PDM limit, the 1.333 MHz limit came from Nordic documentation and
a DevZone response stating the PDM peripheral on nRF5340 only supports
16 kHz output.
Did you achieve higher PDM sampling rates with a microphone
actually connected, or just with the peripheral running standalone?
If 48 kHz PDM is achievable with a real mic on nRF5340, that would change
things significantly for me!!
Thanks!

RE: Audio capture on nRF5340-DK with external I2S ADC (PCM1808) - MCK signal quality and VREF stuck at 0V

Turbo J — Wed, 29 Apr 2026 06:45:26 GMT

MCK: High drive mode must be enabled via pinctrl (overlay) or manually in the CNF register for the pin. Sinuzuidial signal usually means its in standard drive mode. That also applies to PDM CLK.

Some I²S codecs have a PLL that allows the chips to work with a wide range of MCLK inputs. No idea if yours can do that here though, but it would allow you to use an HFCLKAUDIO frequency that satisfies both I2S and PDM requirements.

Where does the 1.333MHz limit come from? I don't see a limit in the electrical specs. I believe I tried up to 48KHz PDM sampling rates but I don't remember if that was with mic hardware attached.