How much is tolerable? Specifically, our product just reaching volume production uses a 9S08QG (again) and has a yield problem. The controller with that uC plugs into a somewhat specialized remote control SMPS, and transmits blinky light signals via SPI to 75HC595s on a control panel. We risked a couple feet (0.7m) ribbon cable for this and a couple other non-critical functions.
On a very few units over a fairly narrow range of SMPS operation, the blinky lights go crazy. SMPS replacement eliminates the symptom. I finally got around to characterizing a bad unit - 20 MHz chirps at the presumed SMPS PWM transitions, growing big enough to upset 3V3 logic as symptoms occur.
Tried some ferrite ribbon cable clamps suggested by the SMPS vendor, supposed to be ~30 ohms at 20 MHz. No effect.
In desperation and with assumptions of 100 ohm pin impedance and 1 MHz OK for a cutoff frequency, we hung 1nF caps between the SPI signals and ground. Symptom eliminated, and the signal transitions are still 2x-3x faster than the recommended '595 maximum (specified at 2V and 4.5V). We have uC pins set to slow transitions & low drive (after seeing overshoot with opposite settings, trying to diagnose this earlier) so could probably reverse that if faster transitions were needed.
This might violate the uC absolute maximum instantaneous output current rating, not by much nor for very long. I wonder what other trouble it could get us into?
Of course I'm seeking support from the SMPS maker toward a fix on that end. Just thought I would ask for advice from the forum before going to the trouble of a board spin...