I will continue discussing the VDD pin before discussing other components. PIC Kit 2 is smart, and does take care of these things, but other programmers may not. Microchip has more then one ICSP schematic around, with and without that connection and other components. It's all quite confusing since there are a million different programmers out there.
As I can see from this PDF that mike suggested on page 4: http://ww1.microchip.com/downloads/en/DeviceDoc/31028a.pdf. There is a resistor between power supply VDD and the ICSP VDD to ensure there is no high current, or is there some other reason for that resistor? Wisp628 requires this VDD to power it's self ?? what if your circuit is 3v? This does not seem to be the correct usage for this pin. This pin is on the programmer to power the PIC/circuit, not the other way around. Again, the voltage difference comes up, now you may have ICSP output VDD of 5v connected to power supply 3v (if you are building a 3v circuit) I could match this ICSP schematic with Jaluino & Wayne's schematic, but I also don't want to risk breaking anyone's programmer, voltage regulator, etc. Isn't it better to be safe? Maybe I simply need an explanation of what this VDD pin is for and when to use it. I think a jumper should be used on this connection. Does this seem like a good solution Joep, Wayne? I'm trying to meet somewhere in the middle to make everyone happy. Matt. On Mar 24, 10:06 am, "m...@watty" <[email protected]> wrote: > On Mar 24, 4:19 pm, Joep Suijs <[email protected]> wrote: > > > If this pinout is what Microchip advises and is commonly used, you > > might have guessed the is no such problem as you describe above. > > Programmers are either capable of handling this (like a pickit2, that > > switches power on and of on this pin) or even rely on power on this > > pin (like wisp628). > > > So please comply. > > > Joep > > The pinout is correct. but the design of the target's PSU circuit must > take account of ICSP feature. That's what I'm talking about. There is > somewhere a Microchip document that outlines the precautions. > > see this ww1.microchip.com/downloads/en/DeviceDoc/31028a.pdf > > also page 4. Which isn't showing the 2nd reverse diode from reset > capacitor to VDD to ensure the capacitor discharges and provides reset > after short power glitch. > > That schematic doesn't number the pins, but is in the correct order. > If the target has only higher impedance devices or peripheral inputs > (PIC is output) on RB7 and RB6, then "Isolation Circuits" are not > needed. If the RB6 or RB7 is inputs from a peripheral in constant > output mode (not tristate during programming) then there should be > jumpers or a pair of switches driven from Vpp / MClR HVprog pin 1. > (13V) > > See alsohttp://www.microengineeringlabs.com/support/icsp.htmand > indication of protection diode, also shown in circuit above. > Even with series resistor, and especially with the enhanced diode > across resistor "reset", you need the series diode to MLCR/VPP pin > from R/C or main VDD to protect your target PSU and reduce draw on the > programming Vpp at 13V nominal. > > see alsohttp://www.embedinc.com/picprg/icsp.htm > Quotes > "Beware that some linear regulators, like common 7805 for example, can > be damaged by raising their outputs above their inputs. A diode from > the regulator output to its input my be required. " > > Professional ICSP Programmer will control VDD. The main PSU must be > somehow isolated or off and only the PIC vdd connected to ICSP. I > accomplish this with two Schottky diodes. > Quote > "When the programmer will drive Vdd during programming, the target > circuit must be off, or at least the power supply to the PIC must be > off. Furthermore the circuit must tolerate the PIC Vdd pin driven up > to the PIC's maximum Vdd spec. This is usually 5.5V, and may be > several volts higher than the circuit normally operates at or is > designed to run at." > > Not all ICSP programmers actually connect to PGM pin > Quote > "PGM should be low during programming > > The PIC PGM pin is used to enable low voltage programming on some > PICs. Even though this is not one of the programming lines and is not > connected to the programmer, it should be held low during > programming. According to the documentation the PGM input should not > matter during high voltage programming, but we have seen cases where > it does anyway. A 100K ohm resistor to ground is a simple fix in most > cases. " > Especially on virgin 16F887. > > Only use a short cable from programmer to target. Mine is about 4cm. > The pod then uses a straight and short RS232 level modem cable from PC > serial port. Poor thin or long Serial cables have too much PGD/PDC > crosstalk. > Quote > "The standard Microchip cable unfortunately puts PGD and PGC on > adjacent lines. Since this is a flat cable, this can and does lead to > crosstalk between the two in some cases. For writing to the target, > the programmer drives both lines. In that case a little low pass > filtering can be applied by the programmer to soften the edges and > reduce the coupled amplitude on one line from an edge on the other. > > However, there is a case that must be addressed by the target > circuit. The PGD line is bidirectional, meaning it is sometimes > driven by the target PIC. In that case PGD is just a normal digital > output on the PIC. These are designed to drive from one state to the > other as quickly as possible. Such an edge produced on PGD by the > target PIC can couple onto the PGC line when using the standard cable > supplied with an ICD2, or any other cable where PGC and PGD are > adjacent. The target PIC then sees a PGC (clock) pulse that the > programmer didn't produce and the serial communication gets out of > sync. The net result is that programming appears to be flaky or not > work at all. > > This entire effect can happen within the time for the PGD edge to > propagate from the target PIC, thru the cable to the programmer, and > back thru the cable to the target circuit. This means that this > problem can not be solved at the programmer end of the cable. No > amount of clever circuitry at the programmer can make this issue go > away. It must be dealt with at the target circuit. " -- You received this message because you are subscribed to the Google Groups "jallib" group. To post to this group, send email to [email protected]. 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