Introduction

Back in the nineties, I was worried if the system could run on 8 MHz. In the end, it did, but now I am curious about what the max speed is. The CPU is a MC68HC000 CPU rated for 16 MHz, but it is unlikely to reach that. The components are pretty slow; the RAM has a speed of 120 ns, and the EEPROM is even slower with 150 ns. The UART and the VIA are old 6800 components and the glue logic is a mix of LS and HCT logic.

Bypass

To test the maximum speed, I had to change the oscillator circuit. Normally, the output of the crystal is divided by two to get an exactly symmetrical output. The  is that the crystal runs twice the processor speed. So for an 8 MHz CPU speed, a 16 MHz crystal is needed. And with simple HC(T) logic, you can't go much faster than that. So I soldered a switch to bypass the divider; the CPU runs at the same speed as the crystal.

Testing

So, I tried:

• 8 MHz: check 
• 10 MHz: check 
• 12 MHz: check 
• 16 MHz: check (!) 
• 20 MHz: check (!!!) 
• 24 MHz: fails

So, the old system runs stable at an astonishing 20 MHz!

How is that possible? Well, the 68000 spends 4 clock cycles for reading/writing a 16-bit word. The successor, 68020, uses only 3 clock cycles, so you could say the 68000 has a built-in wait state. The clock for the UART and VIA is divided by 10, so 20 MHz leads to a clock of 2 MHz, which is the upper bound of the capabilities.

It is amazing that the old system runs so fast!