...MCLR, ICSP and RC time constants.

[PICman]

Hi...

I've been doing PIC programming for a while (began with PIC12C508 and PIC16F84) but never tried In Circuit Serial Programming (ICSP) until now.

On most of my projects, i use an RC time constant with a 1N4148 across the resistor on the /MCLR pin to delay the PIC startup and enable the Master Clear (#fuses MCLR) . i use a 1k resistor between the /MCLR pin and the capacitor.

Now that i want to use ICSP, i find that 1k is fairly low (it will draw 8mA from the Vpp source and will "dump" this 8mA into the Vdd circuit, which i DON'T like !!!). But using a higher value resistor between the RC and /MCLR will render my RC useless since the /MCLR weak pullup (400uA) will put the /MCLR pin HI immediately, regardless of the RC condition.

My question: Is it better to simply remove the RC time constant, use NOMCLR in my code, use a high value (e.g. 100k) resistor between the /MCLR/Vpp pin and either Vdd or Vss and get sure that BROWNOUT is active ?

I've been using the /MCLR RC time constant with the older F84 / C508, but today, i use the newer PIC12F683 and PIC16F88 which have brownout detect circuitry. Are the /MCLR RC time constants "obsolete" ?

Thanks for your help...

[PCM programmer]

I don't use them. All my boards have a +5v or +3.3v regulator on them
for the Vdd power supply (either linear or switching).

Microchip recommends a 10K pullup on MCLR for use with the ICD2:
http://ww1.microchip.com/downloads/en/DeviceDoc/51265e.pdf
See the figure on the right side.

CCS recommends a 47K pullup for use with the ICD-U40:
http://www.ccsinfo.com/faq.php?page=icd_connection

[PICman]

Thanks a lot!

I finally removed my RC time-constant and put-in a 47k resistor between MCLR and Vdd instead. ...Brownout set and functioning (resets the F683 at 2.1 VDC)...

...and as you do, i added a local 78L05 regulator for the PIC.