Update...

Remember this thread?  Well, I tried some experiments with
this -- main code does the initialization and ends (endless
loop).  In the meanwhile, a TMR0 interrupt routine muxes
the display -- it dumps a specific hard-coded pattern to
the ports and cycles this as 3 "columns" of 4 bits.

The result is poor -- even though my calculations tell me
all is okay, and I have verified that I have no situation
where the bits tied together are out of sync with each
other, the resulting display is uneven in intensity, and
incorrect -- some LEDs are on that should not be.  Another
odd thing is that it waits about 5-10 secs before it
starts up, after power is applied.

I added a transistor driver to each of the 3 column lines
and it works properly -- nice bright crisp display, correct
output, and starts up instantly.  And yes, the transistors
were added to the point after the 4 pins were merged
together in each group.  I did this to reduce the # of
changes on each test.

The delay in starting up worries me -- I was worried that
it may be due to a conflict on the ports at startup time
BEFORE the ports got TRISed properly and cleared, but
that would not explain why it started up right away after
the transistors were added.

Can't explain why the poor results though.  I'm wondering
if to switch off the off columns (2 of 3 at any time) by
setting those pins to inputs so they float, vs. setting
them to low.  I'll have to experiment with this tonight.

Cheers,
-Neil.



-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Pic Dude
Sent: Friday, June 14, 2002 10:57 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: [PIC]: Component reduction...


Been trying hard to squeeze my PIC boost gauge (from some months
ago) into a round gauge enclosure, and that means elimination of
as many external parts as possible.  So I'm looking at every
component and thinking of why I really need it.  Converted the
circuit from 10 direct-driven LED's to 12 MUX'ed LED's (3x4 config)
and then this occurred to me last night...

http://www.avn-tech.com/stuff/mux_no_transistors.jpg

Not shown in the pic is that all LED cathodes are to the left.
Also, the dashed line represents the division between the main
board and the display board (yes, I need 2 round PCB's).

I could possibly eliminate the driver transistors since I have
extra ports to generate enough current.  The reason of tying the
lines together is that I can reduce the number of wires going
between the main board and the display board.

I think I'm not violating any PIC rules here:
- I can keep the tied-together lines at the same logic-level by
        keeping them on the same port.
- By multiplexing, only one set of 4 is on at any point in time,
      so I'm within the port's total max current.
- Also within the chip's max current.

BTW, I need the 16F872 since I need an A/D.  Nothing smaller w/flash.

I guess what I'm looking for is the experts' blessing on this.  Is
there something wrong here that I am missing?  Or shall I start
my PCB layout....?

Thanks,
-Neil.

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