18-03-2026, 06:29 PM
For RetroMal,
In the early days, someone would ensure that both the Z80 CPU and PIO were the 4MHz 'A' variants coupled with 300ns fast access BASIC & FONT ROMS (the RAM was already fast enough).
Install a SPDT toggle switch to select between a 2MHz or 4MHz clock & a SPST toggle switch that connects to the CPU's /WAIT line to GND.
To switch speeds, engage the SPST switch to put the CPU on WAIT, change the clock speed toggle switch and then disengage the SPST switch to resume.
The speed switching was handy to ensure that the bit-bashed Tape, Speaker and Serial ports worked normally when needed.
In the early days, someone would ensure that both the Z80 CPU and PIO were the 4MHz 'A' variants coupled with 300ns fast access BASIC & FONT ROMS (the RAM was already fast enough).
Install a SPDT toggle switch to select between a 2MHz or 4MHz clock & a SPST toggle switch that connects to the CPU's /WAIT line to GND.
To switch speeds, engage the SPST switch to put the CPU on WAIT, change the clock speed toggle switch and then disengage the SPST switch to resume.
The speed switching was handy to ensure that the bit-bashed Tape, Speaker and Serial ports worked normally when needed.