Hello everyone, I'm new to this forum and I really need help after a few days of searching I can't manage to find the IC I need.
I have a project with a CPLD, but due to the lack of I/Os I can't do this in an easy way. So my problem is that I have 3 bits representing the number of led powered and I need an IC to convert this number in real led powered. For exemple if I have 000 no led is on, if I have 001 only 1, 010 -> 2 led and so on. I hope I was clear but I'm not really good in English

Tanks you.

EDIT: The truth table should look like my attachment.

 

You need a 3-bit to 8 channel decoder / multiplexer chip.

http://www.nxp.com/documents/data_sheet/74HC_HCT238.pdf

 

Tanks for you fast reply, but there is something that I don't understand, the datasheet says that they are mutually exclusive outputs and what I need is to have the "smaller" output to be high too. How can I acheive that?

 

I can not bring up your attachment. Might try a 74AC138 or 74HC138, 3 line to 1 of 8 decoder. Selected output will be low with about 8 mA drive with Vdd of 6 V.

 

Tanks for your relies, I tink I have found a solution to deal with my CPLD I/Os.

 

What you are searching for is a bar graph type display rather than a 3-to-8 decoder display. There is no single logic chip I know of that will do this. One option is to program a 14 or 16 pin PIC to be a custom logic device; after all, that's what it was invented to do. Did you free up 4 more I/O lines?

ak

 

I suppose you could use an R-2R resistor ladder as an Digital To analog converter then send that analog signal to an LM3914. Set the reference voltage to the LM3914 25% higher than the max voltage of the Resistor ladder max.

The LM3914 is a bar graph display driver that can drive up to 10 LEDs. That is why you need the reference 25% higher than your "DAC". So it only drives 8.

 

If you went with the 1-8 mentioned above you could just use diodes to get a bar from a mutually exclusive output, of course you would need to buffer the output but a line driver could do that... ULN2003A

output 1 drives led 1, output drives led 1 & 2 ..... output 8 drives all 8

That said I agree that a small uP would probably be easier.

 

A MCU can do that easily.

 

I thought this was a dead post but as the truth table finally came up will just add 2 cents on post # 8 by adding some ORs to the 74HC138.
This was used with 4017, active high outputs so with 138, active low outputs, would need some inverters.
Might investigate 74HC259, 8 bit Addressable Latch / 3-to- 8 Line Decoder. Selected outputs high

 

I thought this was a dead post but as the truth table finally came up will just add 2 cents on post # 8 by adding some ORs to the 74HC138.View attachment 105028

Dead? Just a little over 12 hours old!