ps2 – IBM PS/2 keyboard strange waveform

I have a PS/2 keyboard I am trying to connect to a microcontroller. I have noticed that the keyboard behaves differently when connected to a PC as opposed to the MCU.

When connected to a PC, CLOCK and DATA stay high. Key codes are transferred out when keys are pressed.

When connected to the MCU, DATA is pulled low and CLOCK sends a continuous square wave.

I have not found any documentation describing this behavior.

What must be done to properly initialize the PS/2 interface?

I have seen the same behavior with two keyboards one “Cherry” and one “COMPAQ”.

When not connected, both CLOCK and DATA idle high due to their pull-up resistors. There is no code active in the MCU. The square wave is sent by the keyboard.

Below: channel 3 is CLOCK, channel 4 is DATA. You can see that DATA is pulled low. CLOCK is a square wave.

Below: same setup but connected to PC. Inhibit is sent by the PC periodically.