04 Dec 96, Jim Robertson writes to All:

 P> CAREFULL! There is a trap here. The trap is that bit-7 on a 16c621 is
 P> a "reserved bit" and it's programming as a '1' or '0' is unspecified."
 P> Some programmer suppliers, including myself, take the view that the
 P> programming state of UNSPECIFIED reserved bits mirrors the way it is
 P> masked for checksum calculations. This assumption is force on us
 P> because the programming specs are never complete but there is a
 P> logical pattern that corrolates with what information is available.

I assume for reserved bits _any_value_readed_ and use this readed value as mask
for later programming this word. With this assumption I always can work
without errors with all unspecified bits.

 P> (Except for the latest programming spec DS30228F where the specified
 P> programming level for bit-6 on the 16c6x/7x is wrong! If you don't
 P> believe me, check it against the .INC files.)

You mean DS3022F-page 9, fig 3-1, bit 6 for PIC16C62/64/65/73/74 ?
OK, I check .INC files for this processors from (the latest) MPLAB 3.12:

=== Cut ===
; P16C62.INC  Standard Header File, Version 1.00    Microchip Technology, Inc.
[...]
;1.00   10/31/95 Initial Release
[...]
;==========================================================================
;       Configuration Bits
;==========================================================================

_CP_ALL                      EQU     H'3F8F'
_CP_75                       EQU     H'3F9F'
_CP_50                       EQU     H'3FAF'
_CP_OFF                      EQU     H'3FBF'
_PWRTE_ON                    EQU     H'3FBF'
_PWRTE_OFF                   EQU     H'3FB7'
_WDT_ON                      EQU     H'3FBF'
_WDT_OFF                     EQU     H'3FBB'
_LP_OSC                      EQU     H'3FBC'
_XT_OSC                      EQU     H'3FBD'
_HS_OSC                      EQU     H'3FBE'
_RC_OSC                      EQU     H'3FBF'
=== Cut ===
As you can see, bit 6 is always 0, as stated at the Fig-3-1.
The same is for all other PIC mentioned above. I don't see any errors here.
You don't need to program this bit, this bit always reads as "0".

 P> Now, On the 16C711, bit-7 is a code protect bit and if your programmer
 P> programs it as a '0' the following nasties occur:
 P> You can erase it on "JW" parts, ever!

You mean, some of CP bits _can't_ be erased ?
This is documented "feature" for all new Microchip PIC.
You can't erase once programmed CP bit for all new PIC.

 P> So, Don't rush to program your 16c711 as a 16c621, check first the
 P> default state of bit-7. '1' is ok, '0' is bad news.

Correct.

Best regards.

Alexej Vladimirov  avlad@mail.ormix.riga.lv
http://www.ormix.riga.lv/eng/mchip/mchip.htm
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