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Summary – Overclocking the COSMAC CPU’s

August 22, 2017

The video above shows the membership card CPU board that powers the 1802 Olduino running at 8MHZ. The Membership Card normally runs at 1.8MHz but the CPU’s are rated for 5MHz and I had been running the olduino at 4MHz. I built a new CPU card which Lee Hart, the designer, thought would suit the higher speed. The 4093 chip used in the clock oscillator has been replaced with a higher spec 74HC132 and I left a socket for the resonator so I could try various speeds.

I had three CPU chips handy to test with, an RCA branded CDP1802ACE, a Harris branded CDP1806ACE, and an RCA branded CDP1805ACE. These are all 5V parts rated for 5MHz. The 1806 and 1805 have extra instructions and the 1805 has onboard memory but both are pin compatible with the 1802 except for lacking the hardware load mode.

Having loaded a blink program with the 1802 at 4MHz, I tried each chip with 4,6, and 8MHz resonators. The 1802 ran fine at 4 and 6MHz but failed at 8MHz although it seemed to be clocking ok and producing timing pulses. The 1806 worked at 4 and 6MHz but at 8MHz it stopped producing TPA and TPB. The 1805 ran fine at 4,6, 8, and even 10MHz. I think the difference is down to sample variation rather than CPU type or brand but I may source additional parts to try.

In the gallery below there’s a chunk of the membership card schematic showing the pin changes to accommodate the 74HC132 replacing the 4093, images of the top and bottom of the board showing cut traces and jumpers to make the pin changes, and an image of the bottom of the finished board with those changes plus my other changes that accommodate the narrow ROM at U8 and bring signals over to a secondary connector for the olduino card.

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  1. In Which I am Awash in 1802’s | olduino

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