On 5/14/07, Michael Rigby-Jones wrote: > > > >-----Original Message----- > >From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] > >On Behalf Of Vasile Surducan > >Sent: 06 May 2007 08:06 > >To: Microcontroller discussion list - Public. > >Subject: Re: [PIC] ICSP & Vpp-to-Vcc resistor? > > > > > >On 5/4/07, wouter van ooijen wrote: > >> > The resistor pulls up the MCLR pin when the ICSP programmer is > >> > disconnected. If it were only being driven by the ICSP, > >> > disconnecting the programmer would leave the pin floating. > >> > >> But connecting the R to Vcc gives a problem: current into Vcc which > >> might raise Vcc. Why not connect that R to ground, and make it realy > >> high? (let's say 220k). > > > >Wouter, > > > >One of the problems which I'm naming " PIC folklore" is the > >"raising Vcc". The Vcc is generated from a regulator. Any > >regulator has a negative feedback. Any regulator is generating > >the voltage on a load, it has nonsense to keep a regulator > >output unloaded. As long the negative feedback is linear > >comparing a fraction of the output voltage with the internal > >reference voltage, any raise of the output voltage is > >compensated, so the output voltage will be lowed to the > >nominal regulator value by the series transistor inside the > >regulator. The over voltage will be absorbed by the load. > > Most linear regulators can not sink current, so if the load on the regulator is very small (and many PIC based circuits are very low power) then the level on Vdd could be raised by injection of programming current from Vpp. Mike, Which is the level of programming current on most PIC programmers ? If it's comparable with the PIC sinking current than it's right. Remember the linear regulator is not sourcing current when the Vcc is bigger than regulator output voltage. Vasile -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist