# BCD to Seven Segment Display Decoder, I've problem with A to F

Discussion in 'Homework Help' started by Saeed Saeed, Jan 2, 2015.

1. ### Saeed Saeed Thread Starter New Member

Jan 2, 2015
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0
Hello,

I've a project which is basically described as this:

I did this so far: Image

the issue is showing A,B, C, D, E,F

I know that 74LS47N to 7 segment shows the BCD 0 to 9. I don't know how to show the other.
I'm not experenced with any other stuff other than logic gatese, multiplexers, decoders.

2. ### tshuck Well-Known Member

Oct 18, 2012
3,531
675
Write a state table that has a number in binary as an input and 8 outputs - one for each LED in the seven segment display. Forget about the 74LS47.

Column 1 Column 2 Column 3 Column 4 Column 5 Column 6 Column 7 Column 8 Column 9 Column 10 Column 11 Column 12
X3 X2 X1 X0 A B C D E F G decimal point
0 0 0 0 1 1 1 1 1 1 0 0

Where X is the 4-bit input and A-G are the corresponding segments, and a segment in illuminated by a logic 1.

Last edited: Jan 2, 2015
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3. ### WBahn Moderator

Mar 31, 2012
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4,701
The problem is poorly crafted because BCD stands for Binary Coded DECIMAL, meaning that binary values A-F have no meaning. This is why BCD decoder ICs such as the 74LS47 make no effort to decode them (I wish they did) since that would require additional circuitry and make them cost more. They are treated as don't cares and produce whatever output they happen to produce as a consequence of how the ten defined input combinations are handled.

So if you want to display A-F, you need to design the logic to do so (which I'm thinking is the whole point of the exercise).

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4. ### nerdegutta Moderator

Dec 15, 2009
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Have you seen the datasheet for the BCD?

As far as I can see, the 7447 cannot display the letters. But I might be wrong. Attached is a snip from the mentioned datasheet.

• ###### 74LS47.png
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5. ### takao21203 Distinguished Member

Apr 28, 2012
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Why do you say that? All these ICs, these kinds which are still made almost cost the same- next to nothing.
The TTL was designed 40 years ago and there were seperate ICs on purpose.
The HEX decoding ICs did fall into disuse at some point- I wasnt able to get them in a shop 15 years ago already. Nobody except necromancer DIY enthusiasts still mention them.

Before the internet was available it wasnt that easy- you had to mailorder from a catalogue, or inquire shop staff + pay moon prices for the parts. There were some shops in my town, over the years they all closed down except one. Since it was the only shop left, they also did grow. Some B2B offices too apparently- they wouldnt hand out single ICs.

So these kinds of TTL ICs were already in disuse before the rise of the internet, now with help of the same, old parts stockpiles return into circulation. A few kinds are still made actually.

Here on the forum it is tried every so often to resurrect ICs no real distributor will even remember.
There are some such as http://www.unicornelectronics.com/
which do have a stockpile of old ICs but it might be small, with only occasional sales.

Gone are these days of monolithic clock ICs, HEX decoders, bit slice CPUs, PROMS, and individual ALUs as well niceties such as FDCs (Floppy Disk controller).

6. ### takao21203 Distinguished Member

Apr 28, 2012
3,544
461
here's something you should use.

If due to WW3 or something like that you arent longer able to use this technology, you wont be able to use TTL either. Thats what teachers told us- we cant allow calculator, WW3 breaks out, nobody can do maths anymore manually. Troll ministery of education- or was it the ministery of Troll education?

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7. ### nerdegutta Moderator

Dec 15, 2009
2,504
778
Have you seen the movie: Idiocracy?

8. ### takao21203 Distinguished Member

Apr 28, 2012
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not into movies recently

9. ### WBahn Moderator

Mar 31, 2012
17,450
4,701
You actually make my point -- why else where there HEX to 7-seg as well as BCD to 7-seg? I'm not aware of any useful intent behind the A-F decodings of the BCD-7SEG decoders, so why make them when they could have simply used the HEX-7SEG ICs for both?

The additional circuitry costs more. It costs more in area (provided you are not pad-limited, which was not the case decades ago but often is the case today) and it costs more in operating power.

10. ### takao21203 Distinguished Member

Apr 28, 2012
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no wonder white population is declining- forced to wrestle with technology that apparently, is no longer made or sold. While in China, you can buy that STM8 module for \$2.

11. ### WBahn Moderator

Mar 31, 2012
17,450
4,701
What are you babbling about and what relevance does it have to the OP's question?

12. ### takao21203 Distinguished Member

Apr 28, 2012
3,544
461
tell me a serious application where that would make any difference- typically these ICs were deployed in hundreds on large boards.

Didactically significant virtual saving?

13. ### takao21203 Distinguished Member

Apr 28, 2012
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About the fact we did that kind of circuits in college for quite a while, and it was a considerable waste of time. They also did withhold microcontrollers from us, which apparently, they did have available.

Back then, you'd had to pay the equivalent of \$2000 for a development kit, actually.

I havent forgotten it.

14. ### tshuck Well-Known Member

Oct 18, 2012
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Please stop attempting to derail the thread with your anecdotes. Take it to off-topic if you'd like to reminisce to no consequence.

15. ### tshuck Well-Known Member

Oct 18, 2012
3,531
675
Ah, yes, as WBahn noted, BCD does not account for A-F since the decimal in Binary Coded Decimal does not go past 9. I think you want a binary to seven segment decoder.

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16. ### takao21203 Distinguished Member

Apr 28, 2012
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Ok OP needs a 4-input AND, with upto 4 inverters on each of the input, 16 of them, then diodes to OR the outputs to each segment.

I dont get why the learning process should be absent when I do that in software?

Anyway, you can do it hardware, nothing stopping you.
Only one of the AND outputs will be active at any time.

17. ### WBahn Moderator

Mar 31, 2012
17,450
4,701
So your approach would involve 16 four-input AND gates (with ten transistors in each one, assuming CMOS), 32 inverters (with two transistors each), and 80 diodes. So that's 224 transistors and 80 diodes which will give you about four gate delays of propagation delay and be non-cascadable due to the diode drops. And that's not to mention the fact that the outputs will be of the wrong polarity to drive the same type of display, namely a common anode display, that the 74x47 is designed to drive.

Actually, you've just pretty convincingly demonstrated how much of the learning process IS absent when someone just does it in software since the method you propose is way too wasteful and inefficient and is not cascadable. Perhaps if you had paid attention when you did this stuff in college, instead of just telling yourself it was nothing but a waste of your time, you would have known that.

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18. ### crutschow Expert

Mar 14, 2008
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3,072
You could use a MC14495 which is a hex to 7-segment converter that includes A,b,C,d (if you can find one).

19. ### djsfantasi AAC Fanatic!

Apr 11, 2010
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Finding one may be problematic. I searched while preparing my response that I am sure his professor would find a single IC solution unacceptable. My search didn't produce a source. Of course my intent was to make a point and not to procure one. Hence, I may not have looked far enough.
Edit: Apparently I decided not to post that one.

20. ### shteii01 AAC Fanatic!

Feb 19, 2010
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I am getting an impression that you are not supposed to use an integrated chip.