Was playing with 7-segment displays this morning and ran into an =
"issue".
I was able to find a work-around, but wondering how others deal with =
this...

The key to my problem is that I am trying to hook up a few 7-segment
displays to a 16F84A with as few external components as possible (no
external chips is ideal, and as few transistors, etc).  Osc is =
32.768kHz.
RB1:RB7 go to the 7 cathodes (each segment), and RA0:RA3 go to the
common anodes thru indiv. transistor drivers.  From the electrical specs
on the 16F84A datasheet, I should have enough current-sink capability
out of each pin (20ma), but the max for the whole port is rated at =
150ma.
The digit "8" has all segments on simultaneously, so that would exceed
150ma, but I thought that I could split the 7 segments over 2 ports.
BTW, is that 150ma continuous, short-burst peak, or both?  For now,
let's say that I run all 7 segs off of port B.

STRATEGY 1:

    Initialize
    Set digit-select pattern to "0001"
Loop:
    Shift digit-select pattern
    Calculate current 7-seg pattern
    Output to Port B
    Put digit-select pattern into W
    Output to Port A
    goto Loop

The problem here is that I was getting some "ghosting" of one digit onto
the one adjacent to it.  My guess was that at only 32.768 kHz, the time
between retrieving the digit-select pattern and dumping it to Port B was
enough to have the current 7-seg pattern visible on the previous digit.

STRATEGY 2:

Added a delay just before "goto Loop".  At first, I just used a =
255-cycle
delay, but that was so slow, I could count the digits pulsing, so I
optimized the rate where the ghosting was reduced, while not being
able to see the display ficker.


STRATEGY 3:

Switched off all digits (00000000 =3D=3D> Port A) just before setting =
the
output on Port B, then set Port B, then output the correct pattern to
Port A.  This fixed the ghosting.

BUT... I was thinking that if the short period of mixed pattern in the
first pass was enough to cause ghosting, then this short period of
off-time is affecting my other requirement to get the displays as bright
as possible. It looks right, but it just doesn't feel right :-)

Other thought was to kick up the clock to the MHz range and go with
strategy 1.  Ghosting time could be reduced to where it's invisible.

Still sticking with no external chips, how else have any of you dealt
with this?  I know that with external chips, I can do one of...

(a) Use an external decoder chip so 7-seg pattern and digit select
    can be on the same port and latched simultaneously.

(b) Use an external driver chip so that I can get extra brightness
    by (legally) overdriving the LED's

(c) Both of the above with a single decoder/driver chip.


Or should I just leave well enough alone with strategy 3?

Cheers,
-Neil.

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