General • GPIO output not pulling down to 0v

Hi,

I've created an interface between a USB keyboard and mouse and a Commodore Amiga and CDTV. Input signals D- and D+ from a USB breakout board come in to GPIO pins. Output signals to the Amiga come from GPIO pins via a level shifter to the Amiga/CDTV.

When the USB keyboard is connected to the Amiga using just the keyboard data and clock wires (not connected VCC and GND as Pi is powered vi USB) it works and keyboard activity is registered on the Amiga. The keyboard data and clock signals show VPP as 5v on the scope. When plugged into the CDTV, VPP on the scope is about 780mv, it looks like something is stopping the signal being pulled low.

Both Amiga and CDTV use a 10k resistor pack to pull the clock and data signals up to VCC. Measuring the resistance between keyboard clock and or data to VCC on the CDTV reads 0.8k ohms, on the Amiga 1.3kohms. Measuring the resistance between keyboard clock and or data to GND on the CDTV reads 0.8k ohms, on the Amiga 1.3kohms.

USB mouse signal to the CDTV shows the same behaviour, VPP of 700mv or so..... (can't test this on the Amiga as its a different signal, but testing it without connecting to the CDTV shows VPP of 5v).

Any help much appreciated.

Thanks

Statistics: Posted by stevewk — Sun Mar 01, 2026 2:31 pm — Replies 8 — Views 146

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