Strange I2C/TWI Pullup Behaviour NRF53

Question

We're developing on a custom NRF53 board. With the I2C bus disconnected from all peripherals, but with external 10k pullups connected, we get the following waveform when attempring a transaction at 400kHz (yellow SCL, blue SDA): 
 
 It shows the device attempting to communicate (with its system voltage at 3.3V) and seems normal. If we remove the external pullups, however, then we get this: 
 
 As far as we can tell on the software side, the internal pullups are enabled, so its not clear why the waveform would be significantly different (and have strange timing)

Jørgen Holmefjord · Accepted Answer

Hi, 
 bsams said: in some modes the SCL/SDA lines are ties together with 20k 
 Is this a mode on your board, or do you see the nRF5340 tie these lines together? 
 bsams said: can I check if its because the peripheral is waiting for the line to go high again because it thinks the clock is being stretched 
 Unfortunately, there is no events indicating that a clock stretch is ongoing, and there is also no status-register in the TWIM peripheral that will indicate this. I' not sure how else this could be checked. 
 bsams said: and then timing out because its limiting the amount of clock stretching allowed 
 As far as I know, there is no timeout/maximum allowed time for clock stretching. 
 Best regards, Jørgen