On Sat, 18 Nov 2000 20:53:14 -0800, you wrote:

>I am interested in Mike Harrisons idea of trying to get an mpu reset
>circuit (chip) to oscilate, using some resistors and a cap, for my low
>speed external mpu clock.
>
>I need 100Hz nominal, but higher (200, 300) would be okay.
>
>Unfortunatley, I am not really good enough in analog to implement this.
>Could anyone give me some guidelines or some ideas to get started. I =
have
>studied some comparator oscilators but not really sure what I am doing.
>
>Thanks for any help.
>
>John

As a matter of interest, why can't you use an RC osc or 32K crystal?=20

Comparator osc : needs a comparator with push-pull output - most
micropower ones do.

3 resistors from the + input : R1 to ground, R2 to supply, R3 to
output.

=46rom the - input : R4 to output, C to ground.=20

R1=3DR2 - high value to establish a mid-supply reference, say 1M
R3 >=3D R1, determines hysteresis, and the voltage swing on the cap.=20
R4 and C determine frequency

I've not tried a reset chip but I'd start with something like this
(view with fixed pitch):=20

  V+  =20
  |
  R1
  |
  +----------+
  |          |
|---|        |
|   |        |
|Out|--+->   C
|   |  |     |
|---|  R2    |
  |    |     |
  |    |     |  =20
--+----+-----+-- 0v

C charges through R1, until the chip comes out of reset - when the
output goes high, R2 discharges C. (R2<R1)  Reset chips should have
enough hysteresis for this to work. May need some tweaking to get a
reasonable mark-space ration, and you need to use a chip which does
not have a delay (the Microchip MCP ones do).

It may not start if the reset chip behaves strangely at very low
voltages.

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