8085 RESET

Brent Hilpert bhilpert at shaw.ca
Sun Mar 19 14:59:15 CDT 2017

On 2017-Mar-18, at 5:21 PM, Adrian Graham via cctalk wrote:
> 8085-based phone system weirdness continues and I'm beginning to wonder if
> the PSU rails are all coming up in time for RESET to go high - given there's
> 4116 DRAMs in there isn't there supposed to be a proper power up order?
> While I look at using a 20-pin ATX PSU to run this machine temporarily I
> need a safe way to reset the CPU rather than constantly power cycling. The
> RESET line comes from an ICL7611 op-amp via an MC14081B through pins 1-4 of
> a 74LS04 and I need to pull it low for longer than 3 clock cycles.
> I wish I had a schematic to show!

Recalling the messages from December, it looked like proper operation of the reset circuitry at power-up
expected the presence of the battery to power the reset circuitry with 12V prior to the 5V bus coming up.

It looks like you could add a reset switch to the unit by paralleling R406 (11K) with a NO pushbutton switch and ~ 4.7 uF cap (all 3 in parallel).
Closing the pushbutton switch simulates the absence of 5V. The cap provides a little debounce delay, but may not even be necessary.

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