PIC microconroller ICD interface

[zilog]

I am laying the design for a PIC18F-based system, and need to know a few things before I finish up.

I use the Microchip ICD2 programmer/debugger to interface to the chip, but one question remains - is it ok to attach an external open-drain reset source to the /MCLR pin? (pulling actively low) I am afraid that the ICD2 will become unhappy if this occurs, as it pushes 13V at some unspecified current into this pin.

Could someone please enlighten me on this matter?

[BWRX]

I would not connect it directly to the pin. I would use something like an RCR filter to the MCLR pin and then you can put the fet in parallel with the cap. That provides some filtering and current limiting, which are both good things.

[wa4swj]

Hi Zilog,

You typically don't need a reset source. Just tie a 10k ohm resistor from MCLR to Vcc. If you want to use a reset supervisor then here's an app note:

http://ww1.microchip.com/downloads/e...tes/00820a.pdf

Hope this helps. I use an ICD2 successfully all the time with just the 10k pull up resistor.

Best of luck.

Regards,

[zilog]

Originally posted by wa4swj
Hi Zilog,

You typically don't need a reset source. Just tie a 10k ohm resistor from MCLR to Vcc. If you want to use a reset supervisor then here's an app note:

http://ww1.microchip.com/downloads/e...tes/00820a.pdf

Hope this helps. I use an ICD2 successfully all the time with just the 10k pull up resistor.

Best of luck.

Regards,

The external reset comes from the PSU, and is to disable startup in case of bad power. So the problem of using /MCLR still remains.

[wa4swj]

I don't know your application, but once software development is complete you won't be programming it that much, if ever. Why not mount a small switch on the board to select the programming mode when needed?

Just an idea.

[ralphs99]

Hi Zilog,

I don't see any problems. The FET can only be an issue if switched on whilst the ICD2 is connected. The bad power output from your PSU has no software function, and so can be disconnected during program development. (Similar to Ed's suggestion) Just make sure the FET is located near the /MCLR input on the chip to reduce capacitive loading and reduce the likelyhood of noise being picked up. And heavily filter the FET gate.

Cheers, Ralph

[TJNII]

The datasheet for the ICD programmer talks about this very problem. Since it needs to be pulled high, the safest is a pull down resistor direct to the pin, and a diode between the external pull-up and the pin. Then when the ICD pulls the pin to +13 it is blocked by the diode. I believe that is what Microchip recommends, but I haven't read the datasheet in a couple months.