Turn off MCLR weak pull up on pic18F46k22

sandy wilson · Joined: 25 Feb 2004 Posts: 28

Hello,

Can anybody help me with this problem. I am using a max803 supply monitor for controlling the MCLR line on a PIC 18F46k22.
To isolate the mclr line from the max803 for programming I have a diode between the reset line of the MAX803 and the MCLR line of the PIC with the anode on the MAX803 side. I'm using a 22k pull up from the MAX803 reset pin and a 300k pull down on the MCLR line.

My problem is that when I look on the scope the MCLR line is coming up straight away in time to the supply and not waiting for the ~300mS the MAX803 should be forcing it to do. I'm fairly sure it is due to the weak pull up on the MCLR line reverse biasing the diode but for the life of me I cannot see how to turn it off.

I've created a small program to try to turn off the pull ups but no success.

Using CCS compiler 4.130 with the following code:

Ttelmah · Joined: 11 Mar 2010 Posts: 19540

#device ICD=TRUE

This says that the MCLR line is to be used by the ICD.

You can't use the MCLR line as a conventional MCLR whilst also using the ICD.

sandy wilson · Joined: 25 Feb 2004 Posts: 28

Thanks for the quick reply Ttelmah,

tried removing the #device ICD=true section and I'm still getting the same effect.

temtronic · Posted: Wed Jun 01, 2016 8:33 am

hmm.. if using MPLAB, be sure to compile as 'release' and NOT 'debug'. That's another snake that will bite you !

Jay

Ttelmah · Joined: 11 Mar 2010 Posts: 19540

But are you connecting an ICD?.

The unit controls the pin so long as it is connected.

How are you compiling this?.

Are you using MPLAB?.

If so, this will default to re-setting the ICD settings, unless you select for it to compile for 'release'.

The pullup, won't cause your problem. Why should it?. Understand the FET is there all the time, even if turned off, but the pin can be happily pulled down and up (the pullup only delivers about 90uA).

It cannot be turned off. read the data sheet:
"When the RE3 port pin is configured as
MCLR, (CONFIG3H<7>, MCLRE=1 and
CONFIG4L<2>, LVP=0), or configured for Low Voltage
Programming, (MCLRE=x and LVP=1), the pull-up is
always enabled and the WPUE3 bit has no effect."

So the pullup will be enabled, since you have the pin set as MCLR. However your monitor is able to sink 20mA. No problem.

Your diode is your problem. How can the 803, ever pull the reset line 'down'?.

Just connect the 803, directly to the reset line on the PIC, get rid of all resistors, and let the pull-up do it's job.

If you are intending to use an ICD, then have a jumper to disconnect the circuit.

sandy wilson · Joined: 25 Feb 2004 Posts: 28

Ttelmah · Joined: 11 Mar 2010 Posts: 19540

You could do it, but you need much higher currents involved.
You'd need to use something like a 10K pull 'down' on MCLR, which would ensure it is seen as low, even with the internal pull-up in it's most 'extreme' variant (remember always use 'worst case' figures). Then to pull this up to ViH, the reset circuit would need something like a 2K pull-up. Problem then is that you'd still end up needing to have a disconnect jumper to use the ICD, so why not make the circuit simpler and just use the supervisor directly to pull the MCLR down, which is what it is designed to do?.

The supervisor guarantees to start pulling 'down' as soon as the voltage is above 1v (if you look at the graphs, you see a tiny little glitch at this point), so will hold the pin low plenty early enough to ensure the supply has time to stabilise. It's how the circuit is designed to work, and saves parts.....

sandy wilson · Joined: 25 Feb 2004 Posts: 28

The crux of my problem is that can't turn off the weak pull up (should have read the data sheets better). I think I will just remove the diode and pull down resistor and replace the diode with a link to fit after programming and let the 803 do its thing.

Thanks for the help Ttelmah.

Sandyw