7 Seg random numbers..

[John Murdock]

I while back, I gota circuit including a 3 digit segment. Today I had time to make the circuit so I did.

Excluding soldering issues, what factors could cause the the segments to jump up randomly and change randomly showing random numbers?

Take in mind that I have a ribbon cable between the input circuit board (has a switch switch which is debounced) connecting it to the main body with seg, when I move this ribbon cable same issue happens it either doesn't count or shows random number and changes randomly when moved.

Yes seg does have resistors to each LED in it.

If this is less of a common error and more of a 'i have to see you board' problem will take a picture.

 

[Tony Stewart]

Random data failure indicates clocking errors which points to signal integrity issues. Is it clocking and latching the data remotely? Examine grounding, crosstalk, voltage integrity and consider foil or braid shield, interleaved grounds, R terminator or pull up/down on clock to match impedance of driver and ribbon cable ( 120~240 Ohm) to prevent ringing.
or... Design race conditions with data changing on clock edge...with ripple and latency.

If grounding one finger and holding the cable helps, it may be a common mode noise issue related to cross-talk. If the voltage drop is excessive on high current digit drivers, there may be crosstalk from multiplexing. When done and got it working, add polyurethane, silicone or hot glue for cable strain relief near soldered end connections. Same goes for large components like LEDs or big caps. Ensure Vcc is clean and digit driver voltage drop is constant regardless of number of segments active.


etc etc.

In any case it is a design problem.

Are the random digits dim or bright?

If it is a solder issue , it may be sensitive to finger pressure or small shock waves to board. (bump)

 

[John Murdock]

Are the random digits dim or bright?

They are very much bright and alive.

Thanks for your advice will check on that.

 

[John Murdock]

Tony Stewart
The segment now shows the numbers and counts properly but there's some small issues.

It only works when the ribbon cable is in a certain position that takes the full pair of hands i have to do and hence if i were to case it and put it to practical use I'm sure that it is not going to work.

Other than that I realised that the LED segment (g) which goes horizontally across is not lighting up but the soldering seems fine?

 

[Rich D.]

Could be all sorts of things, but my money is on simply a defective ribbon cable. They are IDC ribbons I assume ? It happens a lot when they are moved, weird things happen to circuitry. Usually the conductor opens right at the edge where the ribbon exit's the connector, as a result of being over-flexed, but sometimes there is no hard damage. I've had some that become shorted when the insulation-displacement contacts get bent when being assembled. Failures of the contact area are unusual though, 9/10 times it seems to be the ribbon.

 

[Tony Stewart]

re-crimp the IDC connector, gently in a vice and check for bent pins on the header

 

[AnalogKid]

If the switch is debounced and then the signal goes to the ribbon cable, any mechanical contact problems with the cable and its connectors will look like switch bounce to the receiving circuit.

ak

 

[John Murdock]

If the switch is debounced and then the signal goes to the ribbon cable, any mechanical contact problems with the cable and its connectors will look like switch bounce to the receiving circuit.

ak

ah right.

this explains a lot.

 

[Colin]

You have a high impedance problem.
Add either 100k resistors or 100n caps.

 

[AnalogKid]

You have a high impedance problem.
Add either 100k resistors or 100n caps.

this doesn't.

 

[John Murdock]

If the switch is debounced and then the signal goes to the ribbon cable, any mechanical contact problems with the cable and its connectors will look like switch bounce to the receiving circuit.

ak

I've been thinking about this and have tried and looked into it man times on the net.. can't seem to get round the problem ?

The debounced signal does go to the ribbon cable and into the ICs and eventually the 7 seg but no matter what I try same old things//

I have come to the stage where g the middle seg going across does not light up at all and the following pattern: (soldering for g pin seems fine)

1,2,3,4,5,6,7

10,11,12,13,14,15,16,17

20,21,21,23,24,25,26,27

and so on//

As well as this, it can only start TRYING to count up properly when I hold wires in ribbon cable into a certain place with my 2 hands which seems to change every time. If I dont hold it into this position nothing will happen when i press the count up 4 pin tactile switch.

any ideas?