7-Seg LED Display: Common Anode vs Common Cathode

[valemike]

I've played with a four-digit 7-segment LED display here and there, but only now did I start to consider the current sink/source effects these can have on a PIC.

For those of you who currently use 7-segment displays, would you rather use a Common Anode type, and let each PIC pin SINK up to 15mA of current? Or would you rather use the common cathode type, and let the PIC SOURCE 15mA of current?

The reason why I ask is that, if you read the fine print in the PIC's datasheet, sinking all that current can make you exceed the maximum ratings of "sink" current through Vss. (No wonder I see people who drive their LEDs off the unregulated pre-voltage_regulator supply.)

[Guest]

Neither. I would use a 7-segment driver chip (e.g. 7445 or 42 or something else). If you want to get a reasonable brightness with multiplexed bits then you have to rise the LED current (typically 30-40 mA / segment).

[valemike]

Thanks for the suggestion. However, I need to also display letters on this 7-segment display. Some letters in the alphabet can be expressed best if it were capital, some in lower case, and some not at all (e.g. 'w', 'm', 'x', 'z', etc.).

I think such off-the-shelf display drivers cannot let you light up letters, such as 'A', 'C', 'E', 'F', etc., or can they?

[Guest]

Some of the MAXIM drivers give you access to all the individual segments as well as the decimal point, so you can "roll your own" letters, digits, symbols...

[MGP]

I'll second the Maxim LED driver chips -- I have used literally 10's of thousands of them in medical devices and never had a single failure or problem with them.

They have a lot of benefits, including: SPI interface, brightness control (digital and analog), programmable number of scan digits, and probably some others I'm forgetting. They come in DIP and SMD (SO) packages too.

[dyeatman]

The 7447 will give you from 0 to 9 and A-F for hex

[Humberto]

Depending of the availability of "processor time" to take care of multiplexing the segments,
I preffer to use a driver for each individual lines and "draw" the chars and numbers that
can be represented with 7 segments.
Ex: "0.. to 9" + "A" "a" "b" "C" "c" "d" "E" "F" "H" "h" "o" "u"...
Having a function to multiplexing the output lines, it´s a matter of adjusting the "time-on"
of each segment to get the right brightness and total current consumption.
Maxim´s has drivers with very good features but, at least here in Argentina they
are very expensive for a mass production.
Regarding the 7447 type decoder driver, I don´t like the "old fashion" representation
of the numbers "6" and "9" they does (only with 5 segment)

Humberto


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[valemike]

Never thought of using Maxim for LED driving, but it's a good idea.

However, Maxim's lead times are friggen 15 WEEKS for several devices!!

[rnielsen]

Remember, Maxim is not the only place for semi-conductors. Try going to www.icmaster.com and look there for different parts that you could use.

Using the PIC to drive a few LED's is okay but I would not drive a whole mess of them with it. Always use some kind of a buffer or dedicated driver IC. Let the PIC be the smarts and something else be the muscle.

Ronald

[Kasper]

[drh]

Another led IC to look at is the Toshiba TB62709F.