Hello,
We have been trying to configure the NRF52840 to use the external clock. I am coding with NRF Connect/Zephyr.
We have a custom platform that adds a dedicated 32Khz clock to the NRF52840, connected to pins 0 and 1. When we first received our boards we measured some unusual behavior on the two clock pins. This is what we saw on our scope for the two XTAL pins: (Green signal is C5 and yellow is C4 on schematic)
We are under the belief that the internal clock might be causing some interference. We have tried to configure zephyr using the following in our prj.conf but haven't had much luck. We have tried using different combinations of the following including setting to n. We have found that setting _rc=y does kill the spike on the green signal, but does not create a nice sin wave.
CONFIG_CLOCK_CONTROL_NRF=y CONFIG_CLOCK_CONTROL_NRF_K32SRC_XTAL=y CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y
Our hardware configuration closely matches that of the Adafruit Feather NRF52840 board. Here is our wiring for the crystal. We have since switch to using 12pf caps. The XTAL is the SC32S-7PF20PPM purchased from mouser.
This weird clock behavior is causing instability with the IC. We can program the boards just fine with a JTAG, but run in trouble with running the code. We have also tried completely removing the external clock. This did slightly increase stability, but we still measure the unusual sawtooth like signal.
