This repository contains KiCad sources for a simple single-step controller module for the 6502 microprocessor. It's designed to be connected to the system under test using jumpers. It has a pin header that provides +5 VDC, ground, and the 6502's SYNC and RDY signal paths.
The controller has two push buttons.
- Run/Stop -- is used to toggle between the free running and single-step modes. In the free running mode, the green status LED is illuminated.
- Step -- is used to advance the processor by a single instruction when the CPU is stopped. While the CPU is stopped, the red status LED is illuminated.
The buttons are debounced via the 555 and 74123 ICs to avoid triggering multiple step pulses.
The circuit is designed with a diode such that it can be connected to the 6502's RDY input without disrupting other uses of the signal.
See single-step-6502.pdf for a full description of the circuit design.