Hallo Michael,

my little explaination was addressed to an absolute beginner. I didn't want to make a scientific report. Donald was asking: 'Why to decouple power supply of mcu?' , and I wanted to explain this by the help of ONE mechanism, which can be quite easily understood, even from a beginner.

Data of 50mA 'through current' and time interval of 5nsec is based on several application notes from Philips, Fairchild and Texas Instruments about 74HC-MOS familiy. E.g. in application note 'ESG89001' of Philips you will find data about 'through current' of 50mA for a time interval of 2-3nsec. Specifications change from manufacturer to manufacturer, so I rounded to 5nsec. I stated, that calculations are only a rough approximation in order to be able to estimate in terms of order of magnitude, just for understanding what's going on.
I never stated, that 50mA 'through current' would flow in any inverter stage of any CMOS technology of any MCU. Just for HC-MOS, where I meant 74HC-MOS family. Of course, inverter stages of 74HC-MOS devices are much bigger and much more robust, than inverter stages of mcu, where thousands over thousands of transistors must work. And so, 'through current' per inverter stage of mcu is much smaller compared to 74HC-MOS inverter. But nevertheless through current becomes a relevant factor, even with mcu, because so many transistors are switching.

You asked me to 'differ'. But that's not what a beginner wants to hear. He wants to hear something very simple, so he finally can say: 'What, that's so easy?' He will still have enough time to understand 'fine structure', afterwards.

When Donald fabricates a digital circuit consisting of mcu and digital gate circuitry (74HC-MOS devices) he must achieve good power supply decoupling. May be that for modern mcu he will do this for internal stray capacitance loading, and not for 'through current'. But 74HC-MOS chips he must decouple for 'through current', like manufacturer recommends.

Bye,
Kai