When I learned about microcontrollers, admittedly a long time ago, I wound up thinking that, while there was a wide range of speeds at which the things would run, albeit much more slowly than today's fastest, there was generally a "right" speed. That was a rate that was suited to the task taxed the performance of the MCU and peripherals as little as possible, yet got all the work done within the allocated time budget.

That could be a dozen MHz, or it could be 600 KHz, or whatever.

It's no secret that you can get more done if you go faster. However, going faster doesn't always guarantee getting more done. If you're scanning a keypad, the debounce interval is fixed by the mechanical characteristics of the keypad. If you run faster, you have to waste more time, not that it generally matters, waiting out the debounce time, but, you often have to use more RAM space to maintain the software part of the time delay. It doesn't help to hit the "not ready yet" loop 500 times instead of 50, though it doesn't help much.

If a guy figures out he can do the task with an old part, be it an 8255 or an 8031, it's all the same, isn't it? It doesn't have to run at "modern" speed, so long as it runs at adequate speed.

RE