Multiplexing Seven Segment Display

[thecritic]

I am trying to design a digital clock primarily out of BCD Counter, and BCD to seven segment Decoder. (I can handle the timebase part)
But I would Also like to implement multiplexing for displaying so as to save wire, time and also for the sake of learning. I just spent hours searching but all in vain. What I got was those which use mircocontrollers. But thats out of my knowledge for now.

So, any suggestions using just some MUX ICs would be greatly appreciated.

 

[ericgibbs]

I am trying to design a digital clock primarily out of BCD Counter, and BCD to seven segment Decoder. (I can handle the timebase part)
But I would Also like to implement multiplexing for displaying so as to save wire, time and also for the sake of learning. I just spent hours searching but all in vain. What I got was those which use mircocontrollers. But thats out of my knowledge for now.

So, any suggestions using just some MUX ICs would be greatly appreciated.

hi,
How many digits, eg: hh:mm or hh:mm:ss

 

[MikeMl]

Are you familiar with "Tri-State" or with "Open-collector/Drain" buffers?

 

[thecritic]

Further Details

I would Like to implement up-to HH:MM:SS.
So, in this case, I am led to believe that the time and wire Saving will be enormous. The better part of using Multiplexing is I think not the economy of were saving but the simplicity in using less wires, since less wires to connect less chance to connect incorrectly!

My initial idea is to
1. Generate High Frequency using 555 astable
2. Send this pulse to a 6 bit binary counter (perhaps derived from 8-bit counter)
3. Send the output of this counter to a 1-of-6 Decoder (pherhaps derived from 1 of 10 Decoder)
4. Use the output of the decoder to select which of the six 7-segment decoder to enable.

This part of enabling one of the Display is I think OK.
If I setup my Clock somewhat like this
1. Generate 1 Hz clock pulse
2. Send this pulse to the 6 Cascaded BCD Counters
3. Connect each of the BCD Counters to A single BCD TO 7-seg decoder
4. And of course Make some logic to reset at 60s, and 24.

What I don't know is how to setup things so that only that BCD Counter sends output which corresponds to the Display I selected!

(I have never used Tri-state Logic, But if the problem demands its use, I will be happy to learn, Afterall we do projects to learn new things!

 

[MikeMl]

Your task is to multiplex 6 BCD counters to one 4-line to 7segement decoder. Imagine that you place one hex tri-state buffer with each BCD counter. Two of the bits in each hex tri-state buffer will be unused.
Now wire all six of the corresponding BCD bit tri-state outputs together, and wire them to the corresponding input of the 4-line to 7segement decoder. Do this for each bit of the BCD code.
Now use your 1 of 6 selector to enable each Tristate buffer in turn, meaning that all six BCD digits will get multiplexed to the input of the 4-line to 7segement decoder.

 

[Dragon Tamer]

I'm sorry, but in my mind using an astable 555 is an unreliable way to generate a clock pulse. The tolerances on the resistors and capacitors are likely to make it imposible tto get an acurate time constant. I'm currently working on a digital clock, but it uses an IC out of an old alarm clock. The way my IC keeps track of time is that it uses the 50/60 Hz signal from the transformer's secondary lines. The other most reliable way to create an precission clock signaal is to use a crystal.

 

[thecritic]

thanks

Your task is to multiplex 6 BCD counters to one 4-line to 7segement decoder. Imagine that you place one hex tri-state buffer with each BCD counter. Two of the bits in each hex tri-state buffer will be unused.
Now wire all six of the corresponding BCD bit tri-state outputs together, and wire them to the corresponding input of the 4-line to 7segement decoder. Do this for each bit of the BCD code.
Now use your 1 of 6 selector to enable each Tristate buffer in turn, meaning that all six BCD digits will get multiplexed to the input of the 4-line to 7segement decoder.

Thank you for your reply. I don't know anything about tri-state buffer. But I guess its some sort like this-
1. It has n no. of inputs
2. It has same n no. of outputs
3. When its enable pin is High, It diverts all inputs to corresponding outputs.
4. When enable pin is low, it disconnects (high impedance) the inputs from the outputs.

So, after the buffers are added to each BCD counters their outputs can be joined, provided no two buffers are enabled at the same time right?

I quite got it. Please give me a part number of such buffer. Also, If I can't find the buffer, will this method work to simulate the buffer
1. Find 6 quad 2-input And Gate (to be joined to each BCD counter)
2. join Each of the 4 output of BCD to one of the input of each AND gate of the IC.
3. Tie all the second inputs together. This will act like enable.
4. Add a diode to the output of Buffer(out put of AND gates in this case) So that it will act like High Impedance when not enabled,

 

[thecritic]

Yes thats right

I'm sorry, but in my mind using an astable 555 is an unreliable way to generate a clock pulse. The tolerances on the resistors and capacitors are likely to make it imposible tto get an acurate time constant. I'm currently working on a digital clock, but it uses an IC out of an old alarm clock. The way my IC keeps track of time is that it uses the 50/60 Hz signal from the transformer's secondary lines. The other most reliable way to create an precission clock signaal is to use a crystal.

Yes thats right, I am aware of that. But for now, I am not concerned with that part. I can later manage to use crystal oscillator. (Mains isn't reliable in this part of the world)

 

[MikeMl]

Thank you for your reply. I don't know anything about tri-state buffer....
I quite got it. Please give me a part number of such buffer.

These, among lots of others.

Here is a list of most TTL functions, including those that have tri-state outputs.

 

[thecritic]

Hi, MikeMI
Those ICs seem to Function exactly I expected. I now clearly see how to achieve multiplexing using them. Thanks again.

But, is this the standard way of doing this?? I mean, where did we use multiplexers? Nowhere, right? (We just used a Decoder). Also, we will be requiring 1 buffer per BCD counter and a final 7-seg-Decoder, which isn't much better than just using 1 7-seg-decoder per BCD counter. Where is the material saving??
Isn't there some ICs like BCD counter with Tri-state logic and Enable pin (i.e. combination of Usuall BCD counter and a Tri-state-buffer in single package), at which case it would be much better!

 

[ericgibbs]

hi,
Look at this option, you would need two.
You can get these in 9999 and 5959 versions, CA and CC LED types.

I have used them in the past with no problems.

For reference there are two variants, one for MCU control and the other for manual control.

The datasheet I have posted is poor quality, but if you get the latest Maxim datasheet for this device its much more helpful.

 

[thecritic]

That would be over kill, sir Eric !! I would be left nothing to be done!! I want to do multiplexing myself! (actually this is a college project). I think I should go with those tri-state buffers if I can't find any BCD Counters with feature to set the outputs High Z.

 

[ericgibbs]

That would be over kill, sir Eric !! I would be left nothing to be done!! I want to do multiplexing myself! (actually this is a college project). I think I should go with those tri-state buffers if I can't find any BCD Counters with feature to set the outputs High Z.

hi,
OK, I understand.
When designing your clock, consider early in the design how you are going to set the clock counters to the local 'real' time.
Quite often wanabees design the clock for minimum simplicity only to find they have no method of setting the time.

For the actual counters look at the 74LS92 as well as the 74LS90 , thats if you are going the TTL route or the CD40110B for the CMOS.

Why dont you post a design concept so that we can compare idea's.

 

[thecritic]

hi,
OK, I understand.
When designing your clock, consider early in the design how you are going to set the clock counters to the local 'real' time.
Quite often wanabees design the clock for minimum simplicity only to find they have no method of setting the time.

For the actual counters look at the 74LS92 as well as the 74LS90 , thats if you are going the TTL route or the CD40110B for the CMOS.

Why dont you post a design concept so that we can compare idea's.

Thanks for your help.
I am currently at this stage
**broken link removed**

The last tow sets are intentionally made smaller, for accomodations.
(Sorry for this awkward drawing, But this is what you can get from windows, paint )
I am looking forward to receive some help in order to eliminate the buffers. (since I am highly unlikely to get them here,and also Since I need 6 of them to be wired)

 

[ericgibbs]

hi,
Why dont you download the free expresspcb drawing program, pcb and schematic.?



Ref your circuit, the LED drive scan/mux [anode or cathode type] will have to be kept in synchronisation with the segement scan/select timing.

Also the LED drive CA/CC will have to be faster than 20 refresh rates/sec for each LED else the LED's will appear to flicker. So ensure that the persistence of vision POV is considered.

Can you state which type of logic you are going to use.??

 

[thecritic]

hi,
Why dont you download the free expresspcb drawing program, pcb and schematic.?



Ref your circuit, the LED drive scan/mux [anode or cathode type] will have to be kept in synchronisation with the segement scan/select timing.

Also the LED drive CA/CC will have to be faster than 20 refresh rates/sec for each LED else the LED's will appear to flicker. So ensure that the persistence of vision POV is considered.

Can you state which type of logic you are going to use.??

Thank you for the link to the PCB program.
I didn't quite got what you wanted to ask.
But what I want to ask is is there some work-around or some special counter ICs having inbuilt buffers, so that I won't be requiring those buffers? (I just contacted my local electronics shop about those buffers the answer was negative!)

 

[ericgibbs]

Thank you for the link to the PCB program.
I didn't quite got what you wanted to ask.
But what I want to ask is is there some work-around or some special counter ICs having inbuilt buffers, so that I won't be requiring those buffers? (I just contacted my local electronics shop about those buffers the answer was negative!)

hi,
Unless you really MUST use mux logic etc I would just buy 6 , 74LS247's.
Its going to smaller, cheaper and less hassle to build.

If power consumption is a problem just scan enable the LED anodes.

 

[thecritic]

hi,
Unless you really MUST use mux logic etc I would just buy 6 , 74LS247's.
Its going to smaller, cheaper and less hassle to build.
If power consumption is a problem just scan enable the LED anodes.

Yes I MUST use mux logic. Thats the spirit of this thread. I still think multiplexing would be less hassle because without multiplexing, to wire 7*6 = 42 pins of the display to unique pins of ICs is really a great hassle, the wires would look like a birds nest!

What do you think of this IC 74F579. It has three state output, I can anytime make the output high Z. But I need some way to make this IC work like two BCD counter?

 

[ericgibbs]

Yes I MUST use mux logic. Thats the spirit of this thread. I still think multiplexing would be less hassle because without multiplexing, to wire 7*6 = 42 pins of the display to unique pins of ICs is really a great hassle, the wires would look like a birds nest!

What do you think of this IC 74F579. It has three state output, I can anytime make the output high Z. But I need some way to make this IC work like two BCD counter?

hi,
I will look at that F579 ic datasheet.

 

[thecritic]

Here is the link to the IC
www.fairchildsemi.com/ds/74/74F579.pdf
also see this
**broken link removed**

If I could Utilize these IC as two BCD counter My problem would be solved!