On Sun, Feb 05, 2006 at 01:51:11AM -0800, Jason Harper wrote:
> > the problem with that is you cant ever count "down" with the ROM part
> > IE you cant make a 1 into a 0
> > so its a tally style counter
> 
>   Actually, it's possible to make an up/down counter with a PROM: see the app notes for Dallas/Maxim's "add-only memory" iButtons for details.  Short description: blown bits starting at one end of memory are credits, bits starting at the other end are debits.

Very interesting concept! Also makes for some weird limitations on the
possible values over it's lifetime, a fixed number of transactions, so
your balance can end up wildly in the negative or positive, with a new
card needed to fix it.

If the "midpoint" is fixed you could make a card that over it's lifetime
could only represent fully balanced transactions. To fill it all credits
must match debits...

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