VPP w/ reset chip


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  1. #1
    Join Date
    Nov 2003
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    98

    Default VPP w/ reset chip

    i put a MCP130T-450I/TT reset chip in a design, 'just in case' as the PIC reset line (MCLR) clears a lot of other things too in a fairly busy system
    (this is different project from the other one i have been chattering about)

    when i fired up the hardware i could not program the chip ("target device does not match"),
    I 'scoped VPP and saw it was getting clipped, pulled the reset chip and then it programmed fine

    so three related questions:
    1] i inherited a 0.1uF cap on the MCLR line and i left it in, are the any worries about it slowing down VPP as i notice VPP is pulsing during programming, it seems to work ok, just looking for input from others on this (using MEL U2 programmer)

    2] the reset chip is open drain with an internal 5k pullup, now that i have removed the chip i see that someone else on the 'team' pulled off from the layout the external pullup R i had added in case i did not want the reset chip
    sigh ...
    but then i saw that the line was a nice solid high level, looked at my code and found i had enabled ALL internal pullups for the PIC16F1828 for this project
    so the question here is this
    as the 'weak' pullup is rather weak (100k ??) do you think i should tack on a lower value one ?
    the overall system is large and is too soon to really say that it is working well although at the moment it seems fine as it (I do also have Brown out enabled)

    3] if i had wanted to use the reset chip, what might i have done ?
    this hi voltage VPP is a nuisance compared to other manufacturers (atmel, silabs), but i am not anxious to to go low voltage programming either as that seems to be a different basket of issues

    i have sketched something with some resistors, clamp and steering diodes but it is quite a bit of clutter and parts, putting a jumper on the reset chip would be a royal pain,
    just wondering what others have done here ?
    Last edited by dsicon; - 1st October 2014 at 19:34.

  2. #2

    Default Re: VPP w/ reset chip

    check out low voltage programming.

  3. #3
    Join Date
    Nov 2003
    Posts
    98

    Default Re: VPP w/ reset chip

    my recollection is that for low voltage programming you have to program it for that before you can program it with your code

    also that there may have been other issues or difficulties, are either of those fears true ?

    can you outline the steps to do low low voltage ISP with a virgin blank chip soldered to the board ?
    am using 16F1828 for this one and MEL U2 programmer

    in the meanwhile i made some other progress on the original question here:
    it seems that Microchip makes MCP131 (i was using MCP130, '131 is pin compatible) which is specifically for this issue and has 14V withstand on the open drain,
    they also show a mandatory series resistor, nominal 1k (which means /MCLR pullup would need to be >10k

    this is all good but what i did not mention earlier is that the VPP/MCLR line is also going to a bunch of MCP23017 reset pins which i have isolated for the moment, so i guess i would need a clamp diode on those and hope the U2 programmer can delivered the required current

  4. #4

    Default Re: VPP w/ reset chip

    Sorry, never used that programmer so can't say. Nor am I aware of the downside of LVP (by default I turn off that fuse). I use PicKit2 which has the facility to use LVP, but I have never had occasion to use it.

    George

  5. #5
    Join Date
    Nov 2003
    Posts
    98

    Default Re: VPP w/ reset chip

    what program do you use with the PicKit2 to program chips ?

    i have a buddy who does this stuff in C and uses MPLAB programming environment to program the chips but it makes you compile every time to program the chip, it seems very unwieldy

    re the MCP131 discovery
    the data sheet is a bit of a puzzle, in one place it says -RST ABS MAX 13.5V apparently for the whole family on the data sheet, but in another place it says MCP121 only for 13.5V (the difference is '131 has internal pullup)

    also in related literature it never shows the '131 for this attribute but always the '121
    i have a support ticket at microchip, have never been real impressed with their replies so far but if i find something out i will report back here

  6. #6

    Default Re: VPP w/ reset chip

    what program do you use with the PicKit2 to program chips ?

    i have a buddy who does this stuff in C and uses MPLAB programming environment to program the chips but it makes you compile every time to program the chip, it seems very unwieldy
    I use the GUI based stand-alone, PICkit2V2.exe. It has the handy feature that monitors the .HEX file and if it's modified, will automatically program the PIC. Only downside is that some compilers delete the .HEX file before they try to compile, and of course if compilation fails they do not create a new one. I don't know if PICBasic does this but I know that Proton has a mechanism to avoid this situation.

    If you want to use this program manually, it is not necessary to recompile, just select the .HEX and hit GO.

    George

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