You beat me to it!  Well put.

I would also add that the 595s allow you add more LEDs
(or what ever) in units of 8 by chaining them.  No
real gymnastics needed, just pump the bits out via bit
bang SPI (which is only slightly more complex than
bsf/bcf).  There is a practical limit but its pretty
high.  I used them on an LED light stick design - I've
had as high as 128 working.  The bike light project in
the current Nuts and Volts could have benefitted from
this approach.

Another issue is board space.  A 40 pin PIC takes up a
lot more space than an 8 or 14 pin one.  A 40 pin PIC
seems like over kill to light up 32 LEDs.  

Phil

--- Dwayne Reid <dwayner@planet.eon.net> wrote:

> At 03:47 PM 5/26/2005, Byron A Jeff wrote:
> > >
> > > My favorite answer is a 12F509 + 2 74HC595s (to
> drive
> > > up to 16 LEDs) but I suspect thats not what is
> being
> > > looked for.
> >
> >It begs an interesting question. Why is that combo
> better than
> >a 28 or 40 pin PIC?
> 
> I love questions like this <grin>.  Why indeed?
> 
> My usual responses:
> 
> 1) I can use a smaller packaged PIC (6 or 8 pins if
> the only thing is to 
> read a few switches and drive LEDs).
> 
> 2) I can place the LED drivers near the LEDs and
> connect with only 2 or 3 
> lines plus power and ground.  Handy if the PIC is
> located elsewhere.
> 
> 3) I can drive the LEDs from an un-regulated supply
> and keep the PIC supply 
> nice and clean.  (this is a trick answer: unreg
> supply is about anywhere 
> from 3.3 to 6V if using 74hc595 or just about
> anything at all if using 
> TPIC6595)
> 
> 3b) Can drive reasonable sized relays instead of
> LEDs if using TPIC6595.
> 
> 4) 74hc595 are just SOooo darned inexpensive!  Drive
> 8 LEDs for 10 cents or so.
> 
> 5) One of my boards has 40 status LEDs on it as well
> as 3- 7 segment 
> displays.  Also has 16 or so digital inputs (both
> binary and trinary).  All 
> driven with only 3 i/o lines from PIC.
> 
> 
> No question: if this is a one-off hobbyist project
> that needs only 12 LEDs, 
> spend the extra dollars and go for a larger PIC.
> 
> But **I** would probably do it with a tiny PIC and
> shift registers simply 
> because that's what I'm used to doing for larger
> projects.  And I just 
> happen to have several hundred (or more) little (1"
> x 3") single-sided PCBs 
> that hold an 8 pin PIC, simple 5V zener PSU (with
> JFET discharge circuit), 
> 2- trimpots wired for RC time measurements, 3-
> digital inputs, 2- TPIC6595 
> shift registers.  The PIC section can be snipped
> off, leaving the remainder 
> of the board with the 2- shift registers to
> connected to previous boards to 
> make a SR chain as long as needed.   They get used
> for all kinds of simple 
> projects.
> 
> dwayne > 
> -- 
> Dwayne Reid   <dwayner@planet.eon.net>
> Trinity Electronics Systems Ltd    Edmonton, AB,
> CANADA
> (780) 489-3199 voice          (780) 487-6397 fax
> 
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