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'[EE]:MPU clock using reset chip and rc'
2000\11\18@115407 by John Pearson

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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

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2000\11\18@131016 by mike

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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?
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.

From the - input : R4 to output, C to ground.
R1=R2 - high value to establish a mid-supply reference, say 1M
R3 >= R1, determines hysteresis, and the voltage swing on the cap. R4 and C determine frequency

I've not tried a reset chip but I'd start with something like this
(view with fixed pitch):
 V+     |
 R1
 |
 +----------+
 |          |
|---|        |
|   |        |
|Out|--+->   C
|   |  |     |
|---|  R2    |
 |    |     |
 |    |     |   --+----+-----+-- 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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2000\11\18@151041 by dre Domingos F. Souza

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>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.

       Simple enough:

       1: Get a small 9v transformer, bridge it's output and feed a schmidt trigger. Don't forget to put a 5.1 zener and some R (maybe 220R) to ground to "estabilize" the tension. You have your super-duper-120Hz clock.


--------------8<-------Corte aqui-------8<--------------

       All the best!!!
       Alexandre Souza
       spam_OUTxandinhoTakeThisOuTspaminterlink.com.br
       Linux User #85093

--------------8<-------Corte aqui-------8<--------------

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2000\11\18@231321 by staff

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John Pearson 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


Why not just use the RC clock with a big C??

From what I know of the RC clock driver from the datasheet,
looks like the standard C charged through R and discharged
by the open-drain fet in the PIC when the voltage reaches
the threshold. Just like a 555 timer etc. I can't see any
reason you couldn't just use a 100uF cap and have a 1Hz
clock speed if you like, and 100Hz should be fine too.

You MAY need a series R in the OSC1 circuit to reduce
discharge current but I am not sure if the PIC does this
itself anyway. All guesswork really! :o)
-Roman

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2000\11\19@055029 by mike

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On Sat, 18 Nov 2000 18:13:55 -0200, you wrote:

>>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.
>
>        Simple enough:
>
>        1: Get a small 9v transformer, bridge it's output and feed a schmidt trigger. Don't forget to put a 5.1 zener and some R (maybe 220R) to ground to "estabilize" the tension. You have your super-duper-120Hz clock.
>
..but WHY BOTHER!!!!!! If you have mains, power is not an issue so
there's no need for a slow clock. If you need to sync to it for an
accurate clock a resistor does the job fine.
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