How to use MCLR pin?

young · Joined: 24 Jun 2004 Posts: 285

every chip has a MCLR pin, what this pin used for, can I use it a general I/O to receive and output data, if it is possible, how to check if MCLR is on/off and how to send high and low to it?

rnielsen · Joined: 23 Sep 2003 Posts: 852 Location: Utah

The MCLR pin is used as an external reset to the PIC. Some PIC's offer the option to use this pin as a regular I/O, usually programmed with the #fuses statement. The PIC, itself, does not 'drive' the MCLR pin but is pulled Low whenever the circuit needs to reset the PIC like some kind of supervisor circuit. The PIC can, however, reset itself via a Watchdog in case the proram 'hangs'. Read the data sheet to see if the MCLR pin can be used as a general I/O. You can, also, enable or disable the MCLR input on many PIC's.

Ronald

young · Joined: 24 Jun 2004 Posts: 285

for 12f675 I found out that GP3 is also the MCLR pin, how to define it as general i/0 and how to define it as MCLR using #fuses as you suggested?

Thank you

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

rnielsen · Joined: 23 Sep 2003 Posts: 852 Location: Utah

In your CCS New Project Wizard, un-selecting the MCLR option will put a NOMCLR in your #fuses statement. Try this and then click on the 'View Code Generated' tab to see what is generated for the .h file. These are the 'fuses' that will be programmed when you 'burn' the code into your PIC. The data sheet looks like the GP3 pin is enabled to be an Input-only when the MCLR is disabled.

Guest · Posted: Wed Jul 28, 2004 1:02 pm

Thank you for your great help!!! I am kinds of like a blind person walking on the street need all detailed instructions. I appreciated.

arunb · Joined: 08 Sep 2003 Posts: 492 Location: India

Hello,

I found that the PIC never functioned when the MCLR was left floating, but when I tied it to +5 Volt it became alright.

Is it OK to keep MCLR high permanently??

cheers
arun

rnielsen · Joined: 23 Sep 2003 Posts: 852 Location: Utah

Yes, you can keep MCLR tied 'high' permanently. Never leave it just 'floating' as it will cause you problems. MCLR would be used if you wanted to have an external circuit act as a supervisor and reset the PIC in case things went hay-wire. I simply pull mine high, through a pull-up resistor, and just leave it there. I do, however, use the MCLR input while I'm debugging my programs so that I can reset it without cycling power all of the time.

Ronald