Reply to post: Re: Dynamic logic? - The RCA1802? Great chip

Picking apart the circuits in the ARM1 – the ancestor of your smartphone's brain

Anonymous Coward
Anonymous Coward

Re: Dynamic logic? - The RCA1802? Great chip

Indeed. Slow but reliable as anything. However - 24V 4000 series logic? Nothing I ever used was safe above 15V. The 1802 had a version where the core logic ran at 10V and the interfaces ran at 5, and when I enquired what it was used for the sales guy declined to answer.

While we're completely off topic, let me tell you a story (shut up grandpa). On a system I worked on there was a logic board which kept bringing down its 15V power line. There were, predictably, no schematics. And it did seem to draw a lot of power, with mysterious spikes.

So although it was working we extracted it and worked out the schematic. It was a state machine implemented in 15V 4000 series CMOS. It was a very clever design. And every single unused input had been left open circuit. The board had then been varnished so that each unused input was a little capacitor. It took me a whole day to work out which inputs should be tied to +15 and which should be tied to Vcc. After which it drew 90% less power and the glitch disappeared.

In some circles 4000 series had a terrible reputation for unreliability, and I often wondered how much of this was simply failure to terminate properly.

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