Hi Tony Thanks for the reply. I have some questions about some of the points you made. 1) The RC circuit that Microchip propose for the MCLR/Vpp pin will prevent the Vpp pin from going high for a time dependent on the values of R and C. According to the values I showed in my attachment, this time is 0.1 sec. 2) When you said "keeping pins RB6 and RB7 low", how would you set about this, bearing in mind the low drive capability of a typical PIC programmer? 3) Yes, I have heard about the perils of LVP mode. I'm not using it. However, I suspected that the problem may have been caused by the use of an inappropriate Schottky diode, because the failure to enter programming mode was somewhat random. Another issue may have been my omission of a series resistor into the MCLR pin that was covered extensively in a recent thread. I changed the diode to a common or garden signal type and added a series resistor and, hey presto, reliable programming every time! Alan -----Original Message----- From: Tony Nixon To: PICLIST@MITVMA.MIT.EDU Date: 30 August 2001 08:46 Subject: Re: [PIC]: ICSP MCLR/Vpp issues > Alan Gorham wrote: > > Hi > I'm encountering strange behaviour when programming a 16F874 in > circuit. > I have used a Schottky diode to isolate the programming voltage from > the supply voltage while > programming as recommended in the Microchip technical brief on ICSP. > If I try to modify some code, then the PIC will boot when I try to > program it. > > Has anybody else noticed this behaviour? > > Is it down to the selection of the diode? > > I've checked the voltages on the MCLR/Vpp pin when in normal operation > and while programming and > I see 5V in normal operation and 13V when programming. > > The attached sketch shows what has been done. > > Any hints? It sounds like the chip is not recognising programming mode. Make sure VPP is pulled to GND during powerup to keep the chip in reset. Raise it to VPP when ready to program while keeping RB6, RB7 low. Is the chip in LVP mode? If so, keep RB3 tied to GND as well. -- Best regards Tony mICros http://www.bubblesoftonline.com mailto:sales@bubblesoftonline.com -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu