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From: Peter Jakacki on 24 Feb 2006 04:42
Bill,
What is so complicated about driving an 8-digit LED display? (BTW, you
only wanted 4 digits originally :) ).

Some of the replies you have been getting are basically impractical
(insufficient drive etc) so you are left with either muxing the display
or using dedicated led driver chips. You didn't say whether this is for
indoor or outdoor use etc but I'm guessing that it needs to be bright.

I have used the SAA1064 4-digit LED driver chip before and since it is
I2C is very easy to bit-bash using just 2 I/O from any cpu you care to
name. Two of these suckers would have you home and hosed in no time. Too
simple really.

http://www.semiconductors.philips.com/pip/SAA1064T_N2.html

OK, but they might be a bit hard to get hold of. You can also try the
serial drivers from Maxim etc etc

The mux'd approach is very easy to implement but of course you need
either high-brightness displays or high-current drivers. You figure
which way you want to go. Your software needs to run a low-priority
refresh at around a few hundred to a few kilohertz.

The ULN2003 mentioned earlier is more than fine for segment drive.
Actually, you'd need an octal ULN2803 as a 7-segment display is actually
8 segments including the decimal point. Don't think this handles enough
current? Well, what size power supply have you got? There are practical
limits imposed on every design and if you had to supply that much
current you should look at some other method, even simply using higher
brightness displays.

If this is used to drive the cathode segments you would simply need 8
pnp or pch transistors for the anodes. They don't have to be medium
power, even the good old TO92 packs can handle this easily as long as
they are low saturation pnps or low RDSon pchs. But to drive the source
drivers it is probably advisable to use something like a 74HC138 1of8
decoder to cut down the number of source I/O from 8 to 3. That still
leaves you with 11 lines to drive all that though. If you replace the
2803 with a UCN5822 shift register driver you can cut that down to 5 I/O
plus an optional blanking pin.

Personally, I'd just use a character LCD module, it's so much easier.
You can drive these with as little as 5 I/O (I done it off an 8-pin
PIC). But then again, we are all in the dark (pun) about why you need
the display and how you will be using it anyway. This can totally alter
the whole design choice.

my2cents
*Peter*

http://www.pbjtech.com/


wrongaddress(a)att.net wrote:
>> You could use a CD4511 BCD to 7 segment latch decoder driver, you would
>> need 4 of them and at least 6 I/O lines (assuming external demux) or
>> the TIL311 display with latch $$
>
> Yes, but I have to drive 8 digits and don't want to add 8 chips. I
> thought there might be a 7 segment LED digit with the latch/decoder
> built in, or a group of 4 with the same. That would make it easy.
>
> I can't do it with multiplexing since the duty cycle would be 8 to 1
> and the display wouldn't be very bright.
>
> -Bill
>
From: wrongaddress on 24 Feb 2006 23:24
> I have used the SAA1064 4-digit LED driver chip before and since it is
> I2C is very easy to bit-bash using just 2 I/O from any cpu you care to
> name. Two of these suckers would have you home and hosed in no
> time. Too simple really.


http://www.semiconductors.philips.com/pip/SAA1064T_N2.html


> OK, but they might be a bit hard to get hold of. You can also try the
> serial drivers from Maxim etc etc

They have them at digikey for about $3. Loooks like that may be the
best solution. But the project got put on hold, and customer found
something ready made to use, so we don't need them right now.

Thanks,

-Bill

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