Low slew-rate power might not assert /RESET long enough
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After doing some testing with a range of USB power supplies/chargers I have noticed that one of mine will ramp up the voltage slow enough to prevent the /RESET signal from remaining asserted long enough to properly start the Z80.
This has no impact on the ability to use the reset button.
To 'fix' this, I changed C20 to .47uF. This will keep the /RESET signal asserted five times longer after initial power-up and "works for me" on all the USB power supplies that I have around to test with.
Increasing the size of C20 is OK within reason. Keep in mind that the reset-button (S1) shorts out C20 to reset the system. The switch will not last very long if C20 is large! I'd advise not using anything more than 1uF for C20.
I changed C20 to .47uF in the Rev4 version of the schematic.