Help with circuit(programming pics)

Jon Slaughter · 2007-10-28T20:08:58+00:00

I tried several circuits to program several pics and none work ;/I'm not sure if its the circuit(most likely) or the software(possible but probably not... using WinPic and it has worked for me before(although I could never get it to work with some other chips)).I uploaded the circuit to It consists of a several mosfets.Port VDD(Q2) is simply used to turn on the power to the other mosfets its linked too.VDD, Data, Clock, and VPP are all controlled by the parallel ports control lines(suppose to be open collectors but they have a pullup).I use 13V so I can turn on the mosfets completely but also it is needed to programming the pic in high voltage mode. That part of the circuit is the VPP to PIC VPP(Q1) part. When VPP is high it turns on Q3 which turns off Q which turns off PIC VPP. When VPP is low it turns Q3 which turns on Q1 and hence turns on PIC VPP. I invert the line in the software so its active high.The two sub circuits after Q2 are just "drivers" for the parallel port. Both are non-inverting.To get output from the pic it goes through Pic Data to Data Read(Q5). When the pic drives Pic Data high, Data Read goes low and vice versa. There is a slight issue here in that a Status port pin is attached a to Data Read and it supplies some voltage to the device when it is disconnected. I suppose I can put a diode here to stop that?In any case, everything seems to work when driving LEDS but does not work when trying to program the pic. (I don't use several of the pull down resistors where it shouldn't matter all that much) I invert the lines appropriately and test it all using the software and LEDS.e.g., if I go into winpic and turn on VPP it turns on an led attached to PIC VPP and the voltage is what it should be. Each pin does what its suppose to. (which is that it gives me a low voltage when I send a low voltage in the software and a high when I send a high)I've spent last 3 days working on this and can't for the life of me figure out why it won't program. I've...

Oct 28, 2007 47 Replies

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Jon Slaughter 18 years ago

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Seems to be the DMM ;/ I have a cheap one that I used(cost like 2$) and it seems to somehow be interfering with the signals. When not used I can program a pic chip and but when I hook it up I can't program ;/

My other DMM, which is a much better one doesn't seem to have that problem. (well, its measuring the power supply current so not sure if it would cause the same problem with when used on the parallel port)

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Anthony Fremont 18 years ago

It's loading the pins down thru low-impedance and/or capacitance and screwing up the shape/amplitude of the signal.

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Jon Slaughter 18 years ago

Ok, It seems to mainly be the resistors in the circuit. 2K and below work but above that it doesn't seem to. I'm not sure why such a low resistance is required unless the capacitance of the pins is quite high. (slowing down the transfer rate doesn't seem to help)

It seems that using mosfets are ok except that the when the pic is not being programmed and if there is no shutoff method then it uses an excess amount of current(about 30mA when not programming and 3-4mA when programming).

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Anthony Fremont 18 years ago

There's L in the wires too, not just C. The capacitance of the pin itself is on the order of about 5pF. I keeps tellin' ya, it's all about the rise times. :-)

If the PIC is unpowered and you apply voltage to any pin besides /MCLR (it's special), then current will flow thru the protection diodes and onto the Vcc rail of the PIC (and likely it's associated circuitry). If you're pushing

30mA thru the protection diodes, then you are risking serious damage to the PIC.

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Tom2000 18 years ago

FYI: A Pickit2 programmer is 35 bux from Microchip.

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Jan Panteltje 18 years ago

On a sunny day (Tue, 30 Oct 2007 17:55:45 -0700) it happened Tom2000 wrote in :

Lots of people have been saying this, but to the defense of the OP, I think it is good to write your own prog soft, and build your own hardware. Anyways I went that way too, the experience obtained has more value then 35$. And it was fun to do.

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Vladimir Vassilevsky 18 years ago

Totally agree. Besides, it is much better to learn by fixing the mistakes of your own rather then live with the bugs and the inconveniences created by somebody else.

VLV

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