Cannot get 7 segment displays to work separately.

Thread Starter

veZZ

Joined Jan 12, 2017
3
I've got a cheap PIC board off ebay. It all seems to work fine except the 7 segment displays. I can display numbers upon the first one only or all four together but not separately. I imagine its something to do with lighting one display at a time for a few milliseconds and cycling through them using software.
I've attached the schematic. I cant see how it would work as both IC's are connected to the same PIC pins.
anybody have any idea how it can be done?
 

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AlbertHall

Joined Jun 4, 2014
8,308
I imagine its something to do with lighting one display at a time for a few milliseconds and cycling through them using software.
Your imaginings are correct. U2 and U3 are latches. So you can latch the values for the segments in U2 then latch LED-V1 and display those segments on that display. After a short delay, remove LED-V1 then latch the segments for V2, and finally latch LED-V2 to dis[lay those segments on V2. Repeat for V3 and V4. It works out to be a fair bit of code.

[EDIT] For multiplexing see http://www.vikipedialabs.com/pdf/7segmentdisplay.pdf
Also read the 74HC573 data sheet to understand how the chip works.
 
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Thread Starter

veZZ

Joined Jan 12, 2017
3
Your imaginings are correct. U2 and U3 are latches. So you can latch the values for the segments in U2 then latch LED-V1 and display those segments on that display. After a short delay, remove LED-V1 then latch the segments for V2, and finally latch LED-V2 to dis[lay those segments on V2. Repeat for V3 and V4. It works out to be a fair bit of code.

[EDIT] For multiplexing see http://www.vikipedialabs.com/pdf/7segmentdisplay.pdf
Also read the 74HC573 data sheet to understand how the chip works.
Thanks for the reply. Looks like I've to do a lot of programming and play around with the timings so.... :eek:
 

joeyd999

Joined Jun 6, 2011
4,239
This technique has a name: multiplexing.

It makes the hardware much simpler at the expense of marginally increased code complexity.
 

ArakelTheDragon

Joined Nov 18, 2016
1,265
This technique has a name: multiplexing.

It makes the hardware much simpler at the expense of marginally increased code complexity.
I agree it has a name, like the post of AIbertHall mentioned. But for the rest.

That is not true. The code is simpler too if you use standard transistors and just turn on the display that you need. Unfortunately it takes 1 more pin for every new display.

We can not have everything with no work at all, than engineers would be on the street.

It is best to create 1 function which takes the sign that needs to be displayed and the display on which it should be displayed. That way all you do is give the sign and display name.

Also this is not the whole circuit. Is it common cathode or common anode?
 
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