This is a rant, because I am fed up with the uncertainty surrounding this problem and I want to
clear it up once and for all....

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from just above the highlighted box on pg 136 of the 16f87x datasheet: THIS THEREFORE MAY NOT APPLY
TO ANY OTHER SERIES

"To enter programming mode, VDD must be applied to the RB3/PGM provided the LVP bit is set. The LVP
bit defaults to on ('1') from the factory"

[me: VDD means HIGH by the way]

and in the box below that:

"Note1: The high voltage programming mode is always available, regardless of the state of the LVP
bit, by applying VIHH to the /MCLR pin."

Now that is a _little_ ambiguous but...

LVP   PGM   VPP   mode
0     0     0     normal operating mode
0     0     1     high voltage programming mode
0     1     0     normal operating mode
0     1     1     high voltage programming mode
1     0     0     normal operating mode
1     0     1     ????
1     1     0     low voltage programming mode
1     1     1     high voltage programming mode


where LVP = 0 means the bit is cleared
      LVP = 1 means the bit is set; AS SHIPPED
      PGM = 0 means 0v/pulldown on RB3
      PGM = 1 means 5v/pullup on RB3
      VPP = 0 means 5v on /MCLR
      VPP = 1 means VIHH on /MCLR


there would appear to me to be only one state that is vague.
???? is either high voltage programming mode if you believe Note1 or normal operating mode if you
have more faith in the first statement.


Why on earth do people keep suggesting tying RB3/PGM to ground ? if you tie it to VDD (i.e. HIGH
i.e. pullup) then you don't get to the ???? state and you have no uncertainty.

If the above truth table is wrong for the 16F87X then please someone who _KNOWS_ post a corrected
version and I will get in touch with my local rep and question them about it.
I am not interested in anyone's OPINION of what the ???? state SHOULD be. I just don't care. I want
to know what it IS.

Let's find out and then add a nice clear section the piclist FAQ, and get in touch with Microchip
and ask them to clarify their datasheets.

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Thankyou and my apologies for the rant.


Simon

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