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'[PIC]: Unreliable POR'
2001\03\07@033322 by Kevin Olalde

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I've built my first PIC16F84 circuit, simple counter displayed on a 7
segment LED.  Everything seems to work, but if I power down then power
up the circuit, it seems to pick up right were it left off.  Rather than
starting at the beginning of the program and displaying '0'.  Sometimes
it seems to reset on power up, but not usually.

I've looked at app note 607, but frankly I don't know how to measure the
Vdd rise time (I'm _very_ new to this).  And if it was too slow I don't
know how I'd go about holding the /MCLR pin low.

Any advice would be appreciated.

Thanks,

Kevin

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2001\03\07@042024 by Vasile Surducan

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The most common solution is to do a fast reset circuitry:
10K from MCLR to vcc
100n from mclr to gnd
diode, anod to mclr, cathode to vcc for a fast capacitor discharge
after power down.
( only with a tipical power source with very close to zero output
impedance )

by, Vasile

On Wed, 7 Mar 2001, Kevin Olalde wrote:

{Quote hidden}

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2001\03\07@091552 by Dale Botkin

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On Wed, 7 Mar 2001, Kevin Olalde wrote:

> I've built my first PIC16F84 circuit, simple counter displayed on a 7
> segment LED.  Everything seems to work, but if I power down then power
> up the circuit, it seems to pick up right were it left off.  Rather than
> starting at the beginning of the program and displaying '0'.  Sometimes
> it seems to reset on power up, but not usually.
>
> I've looked at app note 607, but frankly I don't know how to measure the
> Vdd rise time (I'm _very_ new to this).  And if it was too slow I don't
> know how I'd go about holding the /MCLR pin low.
>
> Any advice would be appreciated.

I have seen the same irritating thing.  How long are you powering off?
The bench power supply I use has large output filter caps.  After I turn
the switch off, it will still power a typical small PIC project for a good
5 to 10 seconds, less if there are a few LEDs.  Since the PIC will run on
some very low voltages, it may not reset until well after the LEDs have
stopped glowing.  You may want to try a switch that actually disconnects
the PIC from the power supply rather than just turning off the power
supply input.

Dale
---
The most exciting phrase to hear in science, the one that heralds new
discoveries, is not "Eureka!" (I found it!) but "That's funny ..."
               -- Isaac Asimov

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2001\03\07@094047 by t F. Touchton

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part 1 3456 bytes content-type:text/plain; charset=us-asciiI went through this same problem many times.  If the power supply droops to
around .4 to .9V window, then climbs back up to operating voltage (2.5 - 5V)
quickly (or slowly) it seems that only parts of the PIC reset and operation will
be erratic, if at all.

The moral of the story is to hold the reset pin low when Vcc drops below about
1.8V, and to not let this go high until Vcc is above 1.8V (for parts running on
2.5V).  Haven't had a problem on over 1million PIC's (16C54's) when I make sure
this happens.  I implemented this with a voltage detector.

On other designs... I made sure the power supply depleted very quickly when
power was removed... thus minimizing the chance of powering back up while in the
.4 to .9V window.  (Basically... you couldn't power down and back up fast enough
to catch it in this window).

This is called brownout... and a hard reset will recover if it happens (pull
reset pin low, and return high).


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On Wed, 7 Mar 2001, Kevin Olalde wrote:

{Quote hidden}

I have seen the same irritating thing.  How long are you powering off?
The bench power supply I use has large output filter caps.  After I turn
the switch off, it will still power a typical small PIC project for a good
5 to 10 seconds, less if there are a few LEDs.  Since the PIC will run on
some very low voltages, it may not reset until well after the LEDs have
stopped glowing.  You may want to try a switch that actually disconnects
the PIC from the power supply rather than just turning off the power
supply input.

Dale
---
The most exciting phrase to hear in science, the one that heralds new
discoveries, is not "Eureka!" (I found it!) but "That's funny ..."
               -- Isaac Asimov

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part 2 3198 bytes content-type:application/octet-stream; (decode)

part 3 105 bytes
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2001\03\07@110729 by Anand Dhuru

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Kevin, you need to blled the power when u switch off; just connect a 1K
resistor between Vcc and ground.

Regards,

Anand

{Original Message removed}

2001\03\07@230115 by Kevin Olalde

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Thanks, this also provided me with a clean reset everytime.

Kevin

Vasile Surducan wrote:
{Quote hidden}

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