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[AVR] Problem with Led Display Board using shift register 74HC595

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codename25

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Hi,
I have a problem with my led display board I made with 74hc595 and ATmega16. The display is not showing the character properly instead the leds are blinking. I had attached a video link with this post. Please help me.
 

Ahmad Edi Saputra

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i think thats about clock shifting problem, your clock is slow, if you use delay just try to reduce it

i hope it help
 

andig

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Hi,

This is happening to the Left side of the display. Is the entire display on the left side affected or only the top portion ? If 74HC595 makes the row and the columns are scanned by FETs or Transistor then this seems to be a software problem. Since the same 74HC595 is responsible for all the rows.
Which way is the First Shift Register (DataIn to PIN14 of 74HC595 from microcontroller), if it is on the right side (Which I assume), and then being cascaded from pin 9->14, then it might be a problem of signal integrity. The same clock is used by all the shift registers and longer pcb traces induce inductance.
Try:
1. Make sure Decoupling caps 0.1(ceramic)+(1uF/10uF Electrolytic) on the supply pins of 74HC595
2. Terminate the clock line with a resistor value of say 2.2K
3. Check with a DSO if possible
 

mdorian

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Hi , you're dealing with high ground currents there , make sure the you have a good thick ground line in addition to what andig recommended. I had the same problem , the voltage drop on the ground line is higher on one side. Connect the ground , common clock and latch line in the middle to minimize the voltage drop.
 

andig

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Hi , you're dealing with high ground currents there , make sure the you have a good thick ground line in addition to what andig recommended. I had the same problem , the voltage drop on the ground line is higher on one side. Connect the ground , common clock and latch line in the middle to minimize the voltage drop.

Thanks mdorian Very Important. I missed it. Try lighting only the area concerned with the other portions black. This will reduce consumption and drops from PCB traces. If it lights okay then you can figure out that power/gnd issues exist.
 

codename25

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Thanks all for your reply. I just pulled out all the sift registers and put back again and changed my frequency set in the program. For now it looks good but I will surely look into all the suggestions. Also the shift registers are for columns and rows are connected to controller via ULN 2003.
 

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