MCLR pin won't pull low for ICSP

Nora · Joined: 23 Mar 2008 Posts: 50

Hello!
Best thing to be doing on Saturday night is messing around with an ICSP circuit. Grrr.
I'm using a PIC18F2423 and am using it at the 3.3V level.
Using the CCS compiler and MP Lab to program the chip.

The circuit that I have always used for ICSP on 5V chips does not work.

I measure 1.3V at the MCLR when the programmer is plugged in, and get a voltage error from MP Lab when I try to connect.
When I program the chip with a PICDEM plus board, all is well.
So it is not the chip or the ICD 2.

Thoughts?
Nora (my ICSP circuit below)
https://docs.google.com/document/d/1emI-_-8wUFHzbvImM4ZLr8qqrHBwSd4c_TzcXt748VI/edit?hl=en_US

temtronic · Posted: Sun Jul 10, 2011 5:17 am

Sorry, can't access your schematic,Google directs me to the Google Docs homepage....must be a Sunday morning 'glitch'.....grrrr

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

I also can't access the image. Did you observe the requirements for ICSP pins specified in the ICD2 user manual? A popular fault is to have a reset chip driving MCLR without providing a series resistor that allows ICD to pull it low.

Nora · Joined: 23 Mar 2008 Posts: 50

No, not a glitch, I had it set to "private". Not on purpose, obvs.
Also, the schematic as shown is incorrect - +5V should be +3.3V.
There is no 5V supply on my board.

Do I need special fuses to program low-V? It seems that I can program the chip at +5 on the PICDEM Plus board (it doesn't have 3.3 supply) and the chip works in my 3.3 board.
Thanks!
N_N

temtronic · Posted: Sun Jul 10, 2011 9:21 am

Re-read the OP.
You're using an 'F' version of the PIC,. Generally they are '5 volt' only PICs NOT rated for 3.3 volt use, though you might get lucky and have a few work, but NOT guaranteed.

A quick glance at the DC characteristics chart will show this....

The 'LF' version is rated for 'L'ow voltage use though.You shoild be able to program and run them at 3.3 volts.

Nora · Joined: 23 Mar 2008 Posts: 50

Thanks- I just read operating voltage between 2.2 and 5.5 and thought I could use 3.3V. Shucks! In fact, I used the configurator on the Microchip website to select this chip specifically to be 3.3 operation.
Double shucks!

So when compiling do I need this? Because my compiler gives an error. #include <18LF2423.H>
It will only compile without the L.

Also- if the chip is running on 3.3, should it be programmed as LVP on pin B5?

N_N