Hi All,

    I apologize in advance for the long winded post, but it's been a 
while since I've used a rtcc, and I'm trying to figure out which 
approach is the most cost effective.  I know there is an integrated rtcc 
in the PIC24F series parts, and I've already got a design slated for 
using the PIC24 where I need a battery backed rtcc.  I think I can get 
away with the 3.5uA typical, and even the 10uA max current in the 
powerdown rtcc mode because a 100mAH battery would give me around a year 
of timekeeping without power, but has anyone done this with the PIC24?  
I need USB so I'm using a 24MHz primary osc, and I'm assuming that I 
could configure the rtcc on a 32kHz secondary osc, but if this system is 
battery backed, I'm going to have to react quickly to the loss of line 
power to avoid running the primary oscillator off the battery. 

    All of this makes me wonder if I wouldn't be better off cost-wise 
with an off the shelf rtcc that draws much less current (0.25uA, 0.5uA 
max for one I'm looking at), so that besides the management issues, I 
would not have to worry about needing a rechargeable battery.  For 
example, if I use a 0.25uA battery backed rtcc, and a non-rechargeable 
280mAH battery, I could run virtually forever (compared to the lifetime 
of the product).  The cost of the battery is around $1, and the cost of 
the rtcc is around $1.  For that, I get around 1000000 hours of backup 
time.  But, if I use the typical 10uA of the PIC24, I'm looking at 28000 
Hours which is only around 3 years for replacing the battery - of course 
if it was closer to typical, I'd be looking at around 9 years.  Compare 
that to the worst case on the external rtcc of 0.5uA and my $2 solution 
will run off a battery for 45 years.  If I went to a $4 rechargeable 
battery, things get a lot worse since I'm only getting about 30mAH from 
them, and I don't see where this is a real benefit unless you know 
you're going to be powered up at least once per year, and you want 
something to last more than 45 years. 

    Am I way off base with this comparison of cost and current 
consumption?  What's everyone else doing these days?

Thanks,

Tony



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