24 Dec
2016
24 Dec
'16
4:07 a.m.
On 24/12/2016 03:48, "Brent Hilpert" <hilpert at cs.ubc.ca> wrote:
OK, that explanation helps a bit. My brain still has trouble 'slowing down'
how these components work, like when I first dropped from 3GL programming to
assembly. Fortunately there are a lot of tutorials out there.
On a whim I
managed to solder new legs onto the old LM385Z and it works,
giving 1.2V at IN-, but the output is still only 0.2V.
I don't mind admitting I'm stumped :)
The op amp is configured as a schmitt trigger or comparator with hysteresis:
There is no negative feedback so it is operating at full gain and functions
like a comparator.
However there is positive feedback via R412 (*1), this adds hysteresis to the
trip point(s).
(Brief hackneyed, not rigorous, theory of op: As the input differential varies
past the trip point, the output pulls the + input
further above or below the point at which it just tripped, so the inputs now
have to 'overcome' a greater differential
to trip as the input differential varies in the opposite direction.) One input to the (now) comparator is the 1.2V from
V407 regulated down from
some power rail.
The other input is the ~ 1/4 voltage divider down from +5V formed by R398 &
R406 (netting 1.34V @ 5V).
It appears the idea is that as the +5 supply ramps up at power-on the
comparator trip (and hence release of reset) is delayed till the +5 reaches
something around +4.5V.
That's what I figured yesterday which is why I was surprised that
reinstating the LM385Z didn't make it all spring into life.
The ICL7611 is, I expect, a very-low-power
(Intersil's niche) op amp. Together
with the CMOS 4081 the circuit appears tailored for low-power operation.
Is it supplied by the battery?
It may require the battery presence for stable supply at time of power-up to
get reliable reset operation from this power-on-reset circuit.
That's a very good point so now I'm going to have to scrabble around to find
the battery to find out what voltage it was since I removed it ages ago and
may have recycled it. I don't recall any markings on it though, it looked
like a 'normal' 3 terminal NiCAD wrapped in blue plastic. I did take hi-res
pictures of it however.
The 5V input at R395 does head off towards the battery location before it
hits the resistor so I'll trace that out too.
The diode-resistor pairs at the 4081 AND gate inputs
turn the AND gates into
'asymmetric edge delay gates': the resistor together with some capacitance
delays the switching of the gate for an edge of one direction, while the diode
shorts the delay for an edge of the other direction.
Gotcha, I wondered why the diode was there.
If there is no cap at the input, they must be relying
on the gate capacitance
of the CMOS inputs, making for a pretty short delay.
*1: Are you sure that's a 220? resistor? It's awfully small compared to the
impedance of the voltage divider feeding it.
My bad, it's a 1.4Mohm and tests ok.
Thanks!
--
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?