Microcontroller and 7 segment display question.

Thanks for quick reply... What if i want to use a 4 digi 7 segment display to represent 10bits output from ADC. Do I need a mutliplexer?

Thank you very much.

You can multiplex the anodes and cathodes yourself. That depends what other stuff the 8535 does at the same time.

Rene

If you have 7 + number of digits output bits with sufficient current handling (usually not a problem) you can do the segment decoding via a lookup table in software, and the digit multiplexing with a fairly slow interrupt service routine.

With an external decoder (74hct138, etc) you can only need log2(number of digits) plus the 7 segments. A decimal point will cost you an 8th.

--- No. My favorite trick is to use something like an Allegro A6276.

Basically a shift register with constant current outputs which can drive the segments directly, and with only _one_ resistor needed to set the segment currents.

At the µC you only need three IO's to drive the chip's serial data, clock, and latch enable outputs.

Here's a link:

If you don't mind using current-limiting resistors and you use high-efficiency displays, you could use plain ol' vanilla CMOS HC594's or HC595's to do the job.

-- John Fields Professional Circuit Designer

Thank you Rene,

Would you mind to give me some more details.

Thank you John.

Thank you for your useful information.

Eight port pins and one or two passives is about the minimum circuit cost/complexity.

Best regards, Spehro Pefhany

For hobbyists doing one-offs, too bad Digikey doesn't offer the DIP (EA) form in 1's and Mouser doesn't even carry Allegro. Still, I'm glad to know about it, now.

Jon

A hardware or a software multiplexer dramatically reduces the number of leads going to the LED display.

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As so often it depends... And there are almost always more ways to skin a cat. It takes twelve pins, twelve resistors, four transistors, enough time and some software to make the micro do it on its own. If you can fullfill these requirements, you don't need a decoder. You can even save a pin and a resistor if you don't need the decimal point.

As an alternative you can try to find displays with build in decoders/latches, like TIL311. Still requires eight pins or more if you need the decimal points. Can be reduced to three pins if you feed them via shift registers like the old LS164. (You will need two of them for the digits plus one for the decimal points.)

You can even consider to use (four) shift registers to drive the seven segment displays directly. But as the LS can only handle limited current (8mA if memory serves) you may need either buffers or use high efficiency LED displays.

petrus bitbyter