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'[PIC]: 17C766 Serial programming - info for anyone'
2000\07\23@163907 by Dave Roberts

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Hi all

In previous emails, I'd sought help from piclist members over PIC17C766
ICSP.

From the lack of replies, I presume few people have tried it other than
using standard programmers. My thanks to the two people who replied.

For reference, I offer the following in case anyone else gets stuck the way
I did. I hope using piclist in this was as an archive is acceptable and
apologise in advance if not. At least it means the information will exist
for others!

I found that I could not raise TEST to Vihh ( 13 V ). If I did, it consumed
excessive current.

This appeared as a low impedance resistive path to ground from TEST. Devices
from different batches exhibited it.

I checked the datasheet, found internal inconsistencies and in the absence
of other avenues, doubted the spec.

The answer was that voltage overshoot on the rising edge of TEST killed the
chips. My designs use serial programming in the target hardware. Previous
designs such as for the 14000 worked fine. This time wiring effects caused
the overshoot. Specifically, the rising edge overshot by 3V for about 10
nsec - must use slower circuits in the future !

Also, for reference, the datasheet is in error in the following ways

Programing Datasheet DS30274B dated 1998
Main Device Datasheet DS30289A dated 1998

1. Figure 4.8 of DS30274B shows that device clock should be on RA0 whereas
all other references are to RA1. It should of course be RA1.

2. Figure 4.8 also states that clock should not be applied until MCLR is at
Vihh yet all section 7 figues allow for it to precede Test voltage change.
It may precede MCLR.

3. PS10 is specified as 100 usec in section 7.0 and 100 msec in two places
in Figure 4.8 ( also main 17C7XX datasheet DS30289A also says msec not usec
in D114 on page 244 ) but 100 usec in figure 3.3 and section 6.0 table
( P9 ). 100usec is of course correct.

4. Table 5.2 of DS30274B shows in second word bits 0 to 5 as PM2, whereas
the mplab inc file for the 17C766 implies just bit 7 ( and checksum
calculations also imply just bit 7 in the mask values applied ). I am not
sure of the correct answer but believe it is not as per DS30274B.

5. Note 1 to table in section 6.0 of DS30274B is just plain wrong ( read it
through, you'll understand )

Morals:

Don't believe the datasheet
Don't assume Microchip will get back to you quickly ( they seem to mean well
but can be slow )
Check for and remove overshoot ( ringing ) on Vihh in ICSP designs before
plugging the chip in, unless you hate PIC chips.

Dave

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