>On Thu, 21 Mar 1996, Newfound Electronics wrote:
>
>> >Should the circuit contain an extra resistor between vdd and vpp and if yes
>> >what value?
>>
>> Yes. The value can be 4K7-22K. 10K is good. Some people use a diode but a
>> resistor is better as it allow ESD to discharge. ESD on the Vpp pin CAN
>> scramble the memory!
>>
>Hi,
>
>While it will limit any current flow to a fairly harmless level, a
>resistor alone will not prevent the Vpp voltage appearing on the 5 volt
>Vdd supply line. I prefer to use a diode and resistor in series. The
>programmer should hold Vpp to ground before applying the actual Vpp in
>programming mode thus providing an ESD path.

Dermot,

At no stage should there be Vpp applied without Vdd. It is the presence of
Vdd that stops Vpp appearing on the Vdd pin. I would be VERY concerned if
any programmer applied Vpp without Vdd.

Regarding ESD. While the programmer may hold Vpp at ground, what about when
it is not connected? This is when ESD "scrambling" can occur. I know it less
likely if the usual preventative measures are taken, BUT I know it has
happened and it appears to happen at ESD levels that ordinarily wouldn't
harm the rest of the componentry. IMHO the resistor method has no drawbacks
and does serve to prevent such occurences. I can see only positives for it
myself. However, many people do use the diode without to many problems.

BTW  The "victim" in the case I know of had to reprogram 100 16C84 based
boards again. Ouch!

Regards

Jim