Try to build digital clock, need help

[Eric]

I'm trying to build a digital clock for a final project for my high school
digital electronics class, and I was wondering if anyone could help me out.
My idea us to use 3 74HC390s, and then 4 seven segment displays. However I'm
not sure exactly how I would get it working in base 12 and base 60 for time,
and I'm not sure how I would get it to reset when it reached 12:59. I also
am not sure of how I would be able to get a solid once second pulse to run
the timers. Any help would be greatly appriciated.

 

[Jan Panteltje]

I'm trying to build a digital clock for a final project for my high school
digital electronics class, and I was wondering if anyone could help me out.
My idea us to use 3 74HC390s, and then 4 seven segment displays. However I'm
not sure exactly how I would get it working in base 12 and base 60 for time,
and I'm not sure how I would get it to reset when it reached 12:59. I also
am not sure of how I would be able to get a solid once second pulse to run
the timers. Any help would be greatly appriciated.

1Hz pulse, take xtal, divide.
Or take mains frequency and divide.
PIC microcontroller would make it easy...
Would not bother with normal logic.

 

[Tim Wescott]

Solid 1 second pulse: get a 74HC4060; it's a 14-stage counter with a
built-in oscillator. You can hook it up to a 32.768kHz watch crystal and
get 2Hz which you can divide down to 1Hz, or a 1.6384MHz crystal and get
100Hz and divide down with a 'HC390 (_and_ have 100ths of seconds no
less!).

Quick & dirty 1 second pulse: 555 timer. Use a tantalum or ceramic cap, an
aluminum electorlytic won't work at 1Hz. Or set it for 100Hz and use the
same pair of '390s.

Divide by 6: This is a bit harder with the HC390 because of it's
asynchronous clear. One way to make this work is to watch for a six on the
data output and clear the chip when it happens. Your clock will get a
little "blip" of 6, but it'll go back to normal too soon to see. You can
look for the 6 with an AND gate on QC and QB. You should probably stretch
this pulse (with a 555 as a monostable multivibrator) for reliable
resetting.

Reset on 12:59: Don't. Use a counter that goes from 0-11 (with your nifty
divide-by-six), and display 1-12 with it. Then you'll reset correctly.

 

[Walter Harley]

Reset on 12:59: Don't. Use a counter that goes from 0-11 (with your nifty
divide-by-six), and display 1-12 with it. Then you'll reset correctly.

Another tactic used here is to use a counter chip that has 'preset' data
inputs. With this kind of counter, pulsing the 'set' input causes the
counter to assume whatever value is on the 'preset' data inputs. So, when
you detect a 12 on the output (using logic gates), you trigger 'set', and it
changes back to 1 and starts counting up again.

And yet another possibility is to simply use a clock chip, with all this
logic built into it. But you won't learn nearly so much that way

 

[Eric]

Solid 1 second pulse: get a 74HC4060; it's a 14-stage counter with a
built-in oscillator. You can hook it up to a 32.768kHz watch crystal and
get 2Hz which you can divide down to 1Hz, or a 1.6384MHz crystal and get
100Hz and divide down with a 'HC390 (_and_ have 100ths of seconds no
less!).

Quick & dirty 1 second pulse: 555 timer. Use a tantalum or ceramic cap, an
aluminum electorlytic won't work at 1Hz. Or set it for 100Hz and use the
same pair of '390s.

Divide by 6: This is a bit harder with the HC390 because of it's
asynchronous clear. One way to make this work is to watch for a six on the
data output and clear the chip when it happens. Your clock will get a
little "blip" of 6, but it'll go back to normal too soon to see. You can
look for the 6 with an AND gate on QC and QB. You should probably stretch
this pulse (with a 555 as a monostable multivibrator) for reliable
resetting.

Reset on 12:59: Don't. Use a counter that goes from 0-11 (with your nifty
divide-by-six), and display 1-12 with it. Then you'll reset correctly.

Hey thanks a ton for the help. I was just looking at the 74HC4060, and I'm a
little confused on what it does. It looks like a chip that counts in binary
to 16,384, but I'm not sure exactly how you would get it to reset at either
60 or 12 by looking on the fairchild spec sheet. Also, would you suggest I
use 3 of these chips for mins, hours, and seconds? Also, is there a chip
that can run 2 seven segments displays simultaneously, or would i have to
get it to carry using one chip (sorry I forget the number), and then use a
second one?

 

[Tim Wescott]

The 4060 is just to get the clock (1Hz or 100Hz) for the 'HC360s. You still
need to build the rest of your clock after that. I'd concentrate on the
clock itself and worry about a timebase later (but this depends a bit on
what your teacher is enthusiastic about).

The TTL 7-segment display chips all want to take in four bits and display
one segment -- so you have to use one per digit, or use complex sequencing
schemes.

Are you hard and fast committed to the 'HC390? Another poster suggested
other chips which may not be a bad idea -- particularly since I was
forgetting about display chips. To count from '1' to '12' you can make a
'12' detector and use an HC160 -- then you can load the thing to '1' and
never have to worry about that pesky '0'.

 

[Eric]

I'm acutally not really commited to any particular chip. I'm just
brainstorming ideas at the moment. A lot of these chips your talking about
I've never even heard of or used, so I'm open to absolutley any suggestions
on the best way to do this. However I can tell you that using an IC that is
just a "clock" would not be acceptable for the project. I'm using Circuit
Maker 2000 to design the circuit as well, and some of these chips don't seem
to be in that program. As for worrying about the actual clock portion of the
project (which was what I thought you were talking about before, shows how
much I know...), what exactly would you suggest. I'm not familiar with any
chips that count 0-11 (I'm not really familiar with any chips besides a 555
though), so could you maybe suggest one. Sorry if I'm asking too much, but
just reading the stuff you guys write makes me realize that I know next to
nothing about this stuff.

 

[Walter Harley]

And yet another possibility is to simply use a clock chip, with all this
logic built into it. But you won't learn nearly so much that way

Yet another idea for you: Google for "clock schematic", perhaps along with a
couple of the part numbers you're interested in using. You'll probably come
up with a lot. (I tried Googling for "nixie clock schematic", since "nixie
tubes" are a type of display which only hobbyists use these days, and came
up with plenty of hits.)

 

[Tim Wescott]

The challenge of designing with 7400 series logic is finding parts that do
the most for you. Go to the web page of someone who still makes them
(www.fairchild.com is one, I think ON semiconductor or Motorola does as
well). Search for "counter" -- there's lots of selections.

You may also consider using a microcontroller like the AVR or a PIC (made by
Atmel and Microchip, respectively). If you hook it up and write the code it
won't just be applying a "clock" chip and it'll be much more relevant to
today's professional world. Send your instructor to me if he disagrees.

 

[Eric]

Hey I was wondering if maybe you could give me a specific link. I searched,
but most of the schematics I found didn't have the type of chip listed used
or anything. Also, do you know of any schematics that are based on 74xx
chips?

 

[Stefan Heinzmann]

The challenge of designing with 7400 series logic is finding parts that do
the most for you. Go to the web page of someone who still makes them
(www.fairchild.com is one, I think ON semiconductor or Motorola does as
well). Search for "counter" -- there's lots of selections.

Probably the best resource for this stuff is a document from Texas
Instruments (they have a very wide selection of standard logic chips).
See http://focus.ti.com/lit/ug/scyd013/scyd013.pdf

While it does not contain the actual data sheets, it can serve as an
overview of what's available.

You find, for example, that a divide by 12 counter is still available:
The SN74LS92. Whether you can get it at the corner shop is another
question...

 

[Chris Carlen]

The challenge of designing with 7400 series logic is finding parts that do
the most for you. Go to the web page of someone who still makes them
(www.fairchild.com is one, I think ON semiconductor or Motorola does as
well). Search for "counter" -- there's lots of selections.

You may also consider using a microcontroller like the AVR or a PIC (made by
Atmel and Microchip, respectively). If you hook it up and write the code it
won't just be applying a "clock" chip and it'll be much more relevant to
today's professional world. Send your instructor to me if he disagrees.

I agree. There may be a justification for the teacher to want the
project done with real hardwired logic, since it is a digital logic
class. But it would be a good idea to ask if you can use a
microcontroller. You can make a clock with a microcontroller that can
do all sorts of "features" that would be a pain in the butt to do with
logic chips.

And the skills learned will get you started with microcontrollers,
which, should you continue with electrical engineering as a career, will
be essential.

Good day!