The C74-602 is a speedster. It’s native 20MHz rated clock-rate is higher than is traditionally possible with homemade CPUs. This performance is especially noteworthy given that the C74-6502 is a complete and cycle-accurate implementation of the 6502 instruction-set. There are no artificial wait-states, cycle-stretching, or other corners cut in this design. The C74-6502 owes it’s performance principally to the use of fast 74AC logic and an efficient Microcode Pipeline design (among other optimizations).
Speed tests for the C74-6502 were carried out using the comprehensive Klaus Dormann 6502 Test Suite. This ensured that all paths through the CPU’s circuitry were adequately exercised, and the CPU was left to run for several hours at room temperature to confirm stable operation. The maximum clock frequency recorded was 21MHz at 5.5V.
The Frequency Counter readout below shows this measurement, taken while the CPU was executing at top speed without skipping a beat.
C74-6502 CPU 