I have a circuit that has a 24V section and a 5V section, separated by opto-isolators and an isolated DCDC converter.
Both sides of the DC-DC have decoupling caps, and the MCU has one per Vcc pin. There is a internal ground plane pair for 24V/GNDPWR on one side of the isolation gap, and 5V/GND on the other.
There is a switch input (a foot switch with about 2m of cable) on the 24V side that is connected to an MCU by one of the opto-isolators. This switch, when pressed, has quite a lot of bounce and induces a high-frequency "ring" onto the 5V power supply line, (presumably though the DC-DC converter). The ring appears around 100-200ns in length.
In the schematic, C1 is added to try to prevent too much inrush current on the ground plane when the switch is closed, but it has little effect.
The resistor R1 is chosen to limit current in the opto diode, and C4 gives RC of around 0.22ms, so substantially longer than the noise signal.
Disconnecting the opto-coupler output on U1 Pin 4 doesn't make difference, so the noise must be entering across the isolation gap via U2.
This power supply noise seems to be rather upsetting to the MCU circuit, as it causes occasional resets when the switch is pressed (a similar noise signal seems to be imposed on the MCU crystal signal as well).
Switch bounce isn't an issue, as I can deal with that in software, it's just the noise that's concerning.
How to prevent this noise from interfering with the rest of the circuit?