Post by thread:[PIC]: Best pullup resistor value for /MCLR pin?

<Added tag> Be careful with adding a cap to ground on your MCLR line. IIRC, the ICD2 manual discourages this practice because it can affect the timing with which the line is brought to Vpp. If you look in the ICD2 manual, you'll see a schematic which shows MCLR attached to Vdd through a resistor labelled 1k-10k. I like 10k myself; I feel a bit better about a slightly higher resistance between the 13V Vpp and the 5V (or less) Vdd. Mike H. {Quote hidden}> >Hi! > > > >What would be the best pullup resistor value to connect +5V to the /MCLR >to achieve the most stable operation? (I'm > >using PIC18F452) > > > >I've seen circuit diagrams with straight 5 volts, some with really high >resistors, and this given 'ICD ready' circuit > >with 47K Ohm... > > > >Regards, > > Jean-Pierre >It is not hugely critical - there is no 'best value'. >If you want to do in-circuit programming, you need to ensure that the >resistance is high enough not >to upset your programmer. > >Anything from 0 to 100K would probably fine for most applications. Higher >values will increase noise >pickup, so I'd suggest a maximum of 10K unless there is also a cap to >ground. > >-- >http://www.piclist.com hint: The PICList is archived three different >ways. See http://www.piclist.com/#archives for details. _________________________________________________________________ Take advantage of our best MSN Dial-up offer of the year six months @$9.95/month. Sign up now! http://join.msn.com/?page=dept/dialup -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Mike Hord wrote: > IIRC, the ICD2 manual discourages this practice because it can affect > the timing with which the line is brought to Vpp. Good point, and yup, you definitely recalled correctly Mike. Anyone designing for ICSP should definitely read the programming specs for the device they are using since some (not all) of the programming specs on the PIC18F, for example, call out a *maximum* of 1 uS rise time on MCLR/VPP to enter Program/Verify mode. Depending on the output impedance of your VPP source, it won't take much capacitance to violate this spec. Best regards, Ken Pergola -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

This has proved to be a major headache for me on designs which won't work without an MCLR cap. I had a jumper to the cap that I could remove for programming, and that worked for development. one could also try a resistor-cap-resistor to allow the fast risetime at the pin but still filter transients from the power supply or from EMI. It is not just the ICD2 that has this problem - all of the ICSP programmers I have tried also wheeze when a cap is connected to ground from MCLR. FOr most development projects you can dispense with the cap but it is handy in the real world. -- Lawrence Lile Ken Pergola <spam_OUTno_spamTakeThisOuTLOCALNET.COM> Sent by: pic microcontroller discussion list <.....PICLISTKILLspam@spam@MITVMA.MIT.EDU> 12/10/2003 11:41 AM Please respond to pic microcontroller discussion list To: PICLISTKILLspamMITVMA.MIT.EDU cc: Subject: Re: [PIC]: Best pullup resistor value for /MCLR pin? Mike Hord wrote: > IIRC, the ICD2 manual discourages this practice because it can affect > the timing with which the line is brought to Vpp. Good point, and yup, you definitely recalled correctly Mike. Anyone designing for ICSP should definitely read the programming specs for the device they are using since some (not all) of the programming specs on the PIC18F, for example, call out a *maximum* of 1 uS rise time on MCLR/VPP to enter Program/Verify mode. Depending on the output impedance of your VPP source, it won't take much capacitance to violate this spec. Best regards, Ken Pergola -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

> Be careful with adding a cap to ground on your MCLR line. IIRC, > the ICD2 manual discourages this practice because it can affect > the timing with which the line is brought to Vpp. And there are other reasons, which have to do with the capacitor discharging itself into the PIC when V+ is lost. This can kill the chip. Read the datasheets. Wouter van Ooijen -- ------------------------------------------- Van Ooijen Technische Informatica: http://www.voti.nl consultancy, development, PICmicro products -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.