2010/3/21 PICdude <picdude3@narwani.org>:
> This is what I figure is happening here... All math is being done in
> packed BCD format, so when the seconds go from 0x09, then next value
> should be 0x10. =A0But incrementing will take 0x09 to 0x0A, so he tests
> to see if that BCD-clock rollover point has reached the 'A' in the
> right digit by adding '6', which would take it to 0x10. =A0If adding 6
> does roll it over, the DC bit is set, and this is his indication to
> leave it alone now. =A0DC is actually Digit Carry, and gets set when the
> lower nibble overflows. =A0IIRC it does not work for incf, but for addwf
> it does. =A0The instruction set section of the datasheets tell you which
> instructions affect which status bits, and this explains those bits
> better... http://www.pictutorials.com/The_Status_Register.htm
>
> Back to the code, if adding 6 does not cause the digit carry, then he
> undoes the 6-addition.
>
> Now for the rightmost seconds digit, the test value is 6, but for
> leftmost digit of the seconds, the rollover point is when the '5' in
> 0x59 changes to a '6' (as in 0x60). =A0To test for that, he temporarily
> adds whatever will cause 0x60 to rollover to 0x100 (to set the carry
> flag), and rather than doing the math, my guess is that it's somewhat
> self-commenting to use '0x100 - 0x60'.

Ah, now I get more of it. Thank you for the excellent explanation!

I think I'm probably going to write my own code. It won't be nearly as
efficient as Andrew's but at least I'll be able to understand it!

I might actually make mine unpacked as it might make things a bit
easier for display. I've got plenty of free registers so I'm not too
concerned about saving one or two.

Thanks!

Josh
-- =

A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
        -Douglas Adams

-- =

http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist