In putting together a simple '16F877 programming system I've run into
a puzzling problem.  The system programs the device directly (i.e.,
not a bootloader arrangement) via LVP as described in DS39025E,
"PIC16F87X EEPROM Memory Programming Specification".

Here's the situation.  If I program the user program memory space,
followed by the configuration memory and finally the data EEPROM
memory, the data EEPROM doesn't "take".  It seems to read back OK
while in programming mode, but it's not really there where I meant
to put it--it must have been written to and read back from some
other memory location.  (I know it's not there because the
application program doesn't find it where expected).  After
programming the configuration memory I pulse MCLR* before using the
data EEPROM load and read commands, thinking this will cause the
data to be stored from the beginning of the data EEPROM.

If I load the data EEPROM memory first right after entering the
programming mode, then the user program memory and then the
configuration memory, all works OK.  This is a viable solution to the
problem but I'd sure like to know why the other technique fails.

Has anyone already cracked this nut?

Thanks,

Steve

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