|The 1.0 MHz clock board, attached to the back|
of the MP-A CPU board.
Yes, 1 MHz seems glacially slow by today's standards, but this was 1977! And the Motorola and MOS Technology chips of that time could access memory in a single clock cycle, so they were just as fast as the Intel or Zilog chips at twice the clock speed. (Those devices needed two clock cycles, minimum, to access memory and typically ran 2-5 MHz)
Perhaps as a cost-saving measure, the SWTPc MP-A board only has one crystal. This crystal connects to the MC14411 bit rate generator, which requires one specific frequency: 1.8432 MHz. The MC14411 contains a number of divisor networks to offer common bit rates for serial communication.
The designers of the MP-A used one of those outputs to drive the MC6800 at exactly 0.9216 MHz through clock conditioning circuitry. The MC6800 has two clock inputs that must be non-overlapping. This required about three integrated circuits and about 20 discrete components to produce. (Motorola later developed the MC6875 chip for this purpose. The later MC6802 had an on-chip clock oscillator and driver)
Clever, perhaps, but disappointing. It meant that my computer was almost 8% slower than specified.
|Original crystal from my MP-A.|
This meant that I didn't have a 0.921 MHz computer, I had a 0.89 MHz computer. It was 11% slower than 1 MHz.
This would not do.
|Close-up of the clock board.|
|Crude but functional etching.|
I ran that computer for a couple of years in that configuration, and I never had a problem. Those 1 MHz components could work at 2 MHz, although likely not through the entire 0 to 70 degrees C rating for commercial devices.