On Thu, 29 Jan 1998 19:04:27 -0600 John Payson <supercat@MCS.NET> writes:
>>
>> At 17:02 1998-01-29 -0600, you wrote:
>> >Assuming the displays are common-anode, you'd use NPN transistors.
>To drive
>> >a digit, you set its common wire HIGH, set the appropriate segment
>wires LOW,
>> >and let the other lines FLOAT.  If you want decimal points, then
>you can get
>> >9 digits with 9 I/O pins.
>> >
>> Nice and Smart!
>> We got to have different segment maps for each digit, but thats just
>> porgramming


This is a very good idea.  The major problem is that most multi-digit LED
arrays are internally wired with all the segments connected together for
standard multiplexing.  They could not be used with this method.  If you
use seperate LED digit units, then they could be wired as needed.

This would also work well for a moderate number of discrete LEDs
connected to a small number of port pins.
>
>It's not even that bad.  Assuming you have the data for each digit
>stored

I think a general purpose routine could be used to adjust the data from
one table to display properly on all digits.  The 8 digits are:
0gfedcba - digit 7
g0fedcba - digit 6
...
gfedcba0 - digit 0

This data should be applied to the TRIS register (a 0 turns the segment
on, 1 turns it off by setting the pin to input mode).  The port register
would have data of the form 10000000, 01000000, etc.  The 1 bit turns on
the digit.  The 0 bits turn on segments as directed by the TRIS register.

I'll assume the single-one selection byte is already available.  The
problem is then to generate the TRIS register byte from a byte of the
form 0gfedcba.

This is actually easy to do.  Decrement the single-1 byte to form a mask:
01111111 - digit 7
....
00000000 - digit 0
Make a copy of the data shifted one to the left.  For mask bits which are
0, use the shifted data, for ones that are 1 use the original data.
Finally, force the digit drive bit (the 1 in the selector byte) to 0.

For example, consider digit 6.
Selector: 01000000
Mask:     00111111
Data:     0gfedcba
Data sh.  gfedcba0
Result    g0fedcba

I'll leave the writing of PIC code to do this for someone else.




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