[sdiy] Z80 reset time

[John Henson]

Hi Terje,
Obviously in synth repair I see a lot of CPU reset systems.
This is the clue, it's a system. On the average Polysynth, the CPU reset 
is held low (usually reset pins are active low) until the 5V rail is 
stable, the clocks have started and the battery backup RAM has switched 
over to 5V power. When the above conditions are met, the RAM writes can 
be enabled so that no Data is lost on switch on due to out of control 
logic scrambling the Data.
The CPU reset is held long enough for the above things to happen plus a 
bit more spare time just to be sure.
That being said, 2 seconds does seem a very long reset time, I usually 
see between 0.5 and 1 second. Could the timing capacitor be faulty.
I recently worked on a pair of old Lexicon M200 Reverb's which have 
Z80's in them. The reset system in it uses 10 CMOS gates, a dozen diodes 
and transistors and about 40 passive components. This mutes the outputs 
on switch on and off, makes sure the RAM doesn't get amnesia, and then 
allows the CPU to wakeup. It takes 0.8 seconds or thereabouts, but this 
truly is overkill. That being said it works every time.
John.