Internal Oscillator and Internal MCLR

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thecritic

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I need to use MCLR as input pin and I need to use internal oscillator.
Why does MPLAB warns me for trying to doing this?
I hear that if I do so, I can no longer be able to re-program the device as MCLR is internal and programmer can't take control of it. But why would using Crystal Oscillator allow to do so? What is the work-around?

Thanks.
 
Most of the newer chips can handle this if you have a programmer that can do Vpp before Vdd.

Mike.
 
Do, Vpp before Vdd? Does this means, I have to power the Chip through PK3, so as to provide it control on Vdd?
My chip is PIC16F676
 
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Also, Is there some work-around to program the device in its own target circuit. (I can't power the target circuit by PK3). I hear somewhere that introducing delay loop may work. But since, I can always re-program it in Isolated mode by powering through PK3, I am safe to and so will experiment.
Thanks Pommie and Nigel.
 
thecritic said:
Why don't Crystal oscillator pose the same problem? I am curious.
Click to expand...

It does - except you can kill an external oscillator - you can't kill the internal one.

Basically if the PIC starts running, you can't then access programming mode - so you MUST access programing mode before it has time to run. You don't actually need Vpp before Vdd, you just need Vpp before it runs.
 
thecritic said:
Why don't Crystal oscillator pose the same problem? I am curious.
Click to expand...

I think that it is either the start-up time of a crystal oscillator, or a start-up timer inside the PIC, that is only enabled with the crystal oscillator, that holds it in reset for long enough for the programmer to do what it needs to do.
 
Oh! wow. I used Internal Oscillator and Internal MCLR. I separately powered the target. The target was running. I then connected PK3 to the target. Still the PK3 programmes it smoothly.(It warns me everytime though!)
So, it wasn't that bad as it as been publicized! Perhaps, I was Lucky!
 
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As I said, most of the newer pics no longer have this problem.

Mike.
 
It might be an old pic but the PK3 is definitely a new programmer that has circumvented the problem.

Mike.
 
Pommie said:
I don't know how they fixed it but they have.
Click to expand...

I suppose they could have made MCLR accessable even when the pin is set as an input, and not as reset?. Taking it up to 12V could set programming mode regardless of the pins setting?.
 
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