[sdiy] Z80 reset time

[Terje Winther]

Hi John,
Thanks, very good info. Now I know where to search for errors.

Terje


Den 18. nov.. 2012 kl. 23.11 skrev 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.

Terje Winther
terje.winther at wintherstormer.no
http://www.wintherstormer.no/