I'm planning to use a PIC16F688 in conjunction with a device which can
only operate from a 3.3V +/- 10% supply.  The obvious approach is to run
the PIC from the same supply and connect logic lines directly between
the two parts.  However, there appears to be a problem with In-Circuit
Serial Programming (ICSP) of the PIC16F688 in this configuration.

Table 14.7 (page 143) of the PIC16F688 Rev B data sheet states that "Vdd
for Erase/Write" for the program memory must be 4.5V to 5.5V.  This is
different from many other PICs which seem to be happy with anything from
Vmin to 5.5V (i.e. including 3.3V) for programming.  As I see it, this
make it essential to isolate the PIC Vdd supply so it can be raised to
5V during programming without damaging the other device.  I'm thinking
of using a two-way link to connect the PIC Vdd pin either to the 3.3V
supply (normal) or the Vdd pin from the ICSP interface (programming).

I can see a possible consequential problem.  If the PIC is allowed to
start operating in normal (non-programming) mode while the 5V supply is
attached, then the outputs from the PIC may drive inputs of the other
device to +5V, which will exceed the maximum input voltage ratings of
the other device.  Obviously, I don't want this to happen.

As I understand it, ICSP programmers can operate in one of two modes:
"Vpp first" which appears to guarantee that the PIC never enters normal
operation while the Vdd supply from the programmer is energised; and
"Vdd first" mode which doesn't.  However, I don't know whether real-life
programmers support "Vpp first" strictly enough to ensure that the above
problem scenario never occurs.

Unfortunately, my circuit board needs to be very small (one reason why I
want to use the space-saving 14-pin package of the PIC16F688) and so I
don't have a lot of room to add protective components on the logic lines
such as series resistors and zener diodes.  In any case, I'm not keen to
add this extra cost just to cover an occasional re-programming scenario.

I'd appreciate any thoughts on how best to tackle this.

Many thanks in advance.
-- 
Ian Chapman
Chapmip Technology, UK
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