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?
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?
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/en/AppNotes/00820a.pdf
Hope this helps. I use an ICD2 successfully all the time with just the 10k pull up resistor.
Best of luck.
Regards,
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/en/AppNotes/00820a.pdf
Hope this helps. I use an ICD2 successfully all the time with just the 10k pull up resistor.
Best of luck.
Regards,
wa4swj said: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/en/AppNotes/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.
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
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
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.
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