Problem with shift registers

[jboavida]

Hi,

I have a problem that is driving me nuts. I have the following setup:

On ATMEL AVR microcontroller (Mega163) is driving about 60 shift registers in LED boards (to display characters). The boards are quit large, so I have about 3 meters to the last shif register. Signals are clock, data, Latch and Output enable.

Knowing that:

I have 40mA fan out on the AVR. (more that good to drive 60 shift registers)
If I toogle the signals at maximum frequency the AVR permits (about 65Khz) I have perfect defined signals without any noise in every board.
I have 10K pullups on the AVR side.


But when I use the system to shiftout characters I have random bits that turn on and should be off.

When I lower the pull up of the clock signal to 1K the problem almost dissapears. Other signals have no effect.

I dont understand what the problem is because the scope show perfect signals everywhere...

Can someone help please?

Thanks

Joaquim

 

[BrownOut]

If the errors are random, then you might not catch them on your scope. If your instrument has a glitch detection system, you could possible use it to see what's going on. Or you might use a dedicated clock driver between your AVR and hardware. Clock lines need to be very clean, so keep that in mind when designing the connections. Good luck!

 

[jboavida]

The problem only happens after the board number 5. Boards 1 to 5 are ok, boards 6 to 10 have problems. Are you suggesting maybe a LS245 ?

Thanks