Q: using MCLR for Input on 12F683

[johnnylynx]

OK, I have PBP now...i've loaded it up and want to try programming in MPLAB IDE but using PBP.

I used the wizard to create the project, imported a pbp file...but the screen is blank.

Am I missing something? Can one create PBP files in MPLAB??

John.

[mackrackit]

Follow the instructions here.
http://www.melabs.com/support/mplab.htm

Works very well.
Just do not download the latest MPLAB. Use the one that came on the PBP disk.

[g-hoot]

You can set it up for an input from the beginning of the program then still use it later for programming. Just make sure your circuit will still allow the Vpp signal to control it during programming otherwise it will boop at you and report an error.

As a newbie, I was under the impression that the MCLR had to be pulled high when the circuit is powered up, but after reading a little more, I'm not sure. My circuit will have a switch that is either off or on at power up. I'm planning to pull a pin to ground if the switch is on and then I would use internal pull-ups on the pin when it is off. Since I don't program the chips on board, can I use the MCLR pin for the switch, or does it need to be high when the circuit is powered up? That would be awesome if I could free that pin up for use.
Gary

[Melanie]

On the 12F683, MCLR can be handled internally by the PIC (see CONFIG options in the PICs Datasheet), thereby freeing that pin for your use. (equally applies to 12F675 and 16F628 since they were mentioned earlier in this thread).

[g-hoot]

On the 12F683, MCLR can be handled internally by the PIC (see CONFIG options in the PICs Datasheet), thereby freeing that pin for your use. (equally applies to 12F675 and 16F628 since they were mentioned earlier in this thread).

Thank you Melanie,
I looked at the data sheet and see what you are saying. The one thing I'm still not sure about is if the MCLR pin has to be high when the circuit is powered up? If so, then my switch won't work since it may be pulling the pin to ground. This blurb from the PIC Basic Pro manual is what has me confused:

Make sure the /MCLR pin is connected to 5 volts either through some kind of voltage protected reset circuit or simply with a 4.7K resistor. If you leave the pin unconnected, its level floats around and sometimes the PICmicro MCU will work but usually it won=t. The PICmicro MCU has an on-chip power-on-reset circuit so in general just an external pull-up resistor is adequate.

Gary

[Melanie]

If you set the CONFIG for the PIC to handle MCLR internally, then FORGET about MCLR being high or low... it's all handled for you behind the scenes.

Thereafter the pin (that was formally the MCLR pin) is treated as a regular I/O (except that on some PICs it's INPUT ONLY - check with your PICs Datasheet).

If you need the pin to be normally HIGH and it's pulled down by your switch, then you need a PULL-UP Resistor (usually connected between the PIC pin concerned and +5V). Some PICs have internal Pull-Ups you can enable either on a pin-by-pin basis, or globally for an enitire Port. Again check with your Datasheet if the pin you want to use has Pull-Up's available on it - not all pins have them.

If you need the pin to be normally LOW and pulled-up by your switch (not a recommended way of connection - but it will work), then you need a PULL-DOWN Resistor (usually connected between the PIC pin concerned and 0v). You will have to provide one of those yourself externally as the PIC will not provide one internally. Any value between 4K7 and 22K would probably suffice.

[g-hoot]

If you set the CONFIG for the PIC to handle MCLR internally, then FORGET about MCLR being high or low... it's all handled for you behind the scenes.

Thereafter the pin (that was formally the MCLR pin) is treated as a regular I/O (except that on some PICs it's INPUT ONLY - check with your PICs Datasheet).

If you need the pin to be normally HIGH and it's pulled down by your switch, then you need a PULL-UP Resistor (usually connected between the PIC pin concerned and +5V). Some PICs have internal Pull-Ups you can enable either on a pin-by-pin basis, or globally for an enitire Port. Again check with your Datasheet if the pin you want to use has Pull-Up's available on it - not all pins have them.

If you need the pin to be normally LOW and pulled-up by your switch (not a recommended way of connection - but it will work), then you need a PULL-DOWN Resistor (usually connected between the PIC pin concerned and 0v). You will have to provide one of those yourself externally as the PIC will not provide one internally. Any value between 4K7 and 22K would probably suffice.

This is great news! Thank you for explaining it so well.
Gary