comments below

At 04:11 PM 7/9/2003 +0100, you wrote:
>1) Does anyone know a source of a programming adapter suitable for an
>18LF8620 (80pin TQFP 0.5mm pitch) preferably suitable for use with a PS+
>(i.e. 40PIN DIP)?

No, but check Google. Emulation Technology might have one.

>2) I am having a nightmare trying to program the above devices using ICSP.
>I have been trying a Promate2 (with MPLAB 5.74 and MPLAB 6.30) and an ICD2
>(with MPLAB 6.30) and the results with both are the same.  The PIC has to
>run at 3.3 volts, and the power is shared by many devices that are not 5v
>tolerant.

Hmm, I wasn't aware that a PIC could be reliably programmed at 3.3V. I
thought the spec was 5V +/- 0.5V.

>With a brand new PIC, if I perform a blank check, the programmer (either of
>them) reports that the device is not blank.  Specificaly, the program memory
>always has the same pattern in memory starting with 0xC0C0 at address
>0x0000.  If I try to erase the PIC at 3.3volts, it fails to erase the
>program memory, but the PROMATE then reports that the configuration fuses
>are clear.  This is exactly the opposite to what logic dictates as the
>datasheet states that the configuration fuses can only be cleared by a bulk
>erase at >=4.5volts.  In any case, the PIC fails to program.

This is also symptomatic of a LV condition.

>After pulling my hair out for a day, I stared lifting the Vdd pins on the
>PIC so that I could power it from an external 4.5volt supply and perform a
>bulk erase.  After this, the PIC will program happily at 3.3 volts.
>However, this is really not practical as a) I have a stack of boards that
>all exhibit this problem, and b) the PCB is made from a material suitable
>for high frequency use and the copper delaminates very easily compared to
>FR4.
>Has anyone else seen this, and does anyone have any suggestions as to what
>to try next? (Microchip has been contacted, but deadlines are looming!)

I have a standing design technique that ensures isolation for the PIC
itself- I use a diode to isolate the PIC from the rest of the board. In
that way, it doesn't matter what voltage the rest of the PCB uses, the PIC
gets 5V (or 13V on MCLR-) for programming.  I use an 0805 30V Schottky
Diode (50ma) and the  VPP/MCLR pin has a small cap (100pF) and a 22K
pullup, giving the programmer free reign of the chip. Be sure to use a .1uF
cap across the PIC's power pins directly, to ensure reliable normal
operation. The average voltage drop across the diode is about 60mV, as the
PIC doesn't normally use much current. I've never had a problem at any
clock speed.

>Regards
>
>Mike
>
>
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