Hi,
I am trying to read the logic low condition on an external peripheral using F28M36P63C2's GPIO12,13 and 14. There are no decaps on these pins but I am using an external 5K pull up with a 1500 pF filter cap on all these lines. On the scope, I am seeing a ~220KHz noise on these lines, which is very hard to filter. When I connect an oscilloscope probe, especially a differential probe having a 250KOhm input impedance (my circuit is floating in respect to earth ground) , the probe loads the circuit and the target line drops close to logic low. I am seeing this issue on multiple boards where the firmware falsely detects these pins as a logic low. The target peripheral pins have a sink current of 3mA.
I am wondering whether I am driving these pins properly, what is the minimum current that needs to sink to these pins for a robust drive strength . Why are these pins so much susceptible to the noise. What steps can I take to improve the noise immunity of these lines and filter this noise .
Thanks