many TLC5940

[stephanschulz]

Hi.

I made a PCB layout with 12 daisy chained TLC5940 but all my different tests show that only 6 in a row work. As soon as i connect the other 6 i get random LED.

I added the 0.1uF and a 2.2K and followed all other steps mentioned on the Tlc5940 teensy library page.

I tested my PCB by cutting the traces that connect the last 6 TLCs from the first 6 TLCs, then things work fine. I ran all control lines only to the last 6 TLCs and things works.
But when all are connected together the LED do not behave properly.

I read adding other CAPs might help, but this did not see to produce good results either.

Should i somehow change the Clock speed, since maybe the signals do not synch otherwise on such long chains?

I am attaching a screen shot showing my teensy, 3 of the 12 TLCs and the full PCB board.
Thanks for any advice.

 

[stephanschulz]

could it be as simple as not picking 96 MHz but rather 72 MHz for the CPU speed?

 

[Donziboy2]

Try slowing your SPI speed down.

 

[tonton81]

try to add 100ohm series termination to prevent ringing on the long line

 

[PaulStoffregen]

You might need a buffer chip like 74LCX125 to drive the clock and other shared signals to so many pins.

Modern logic families like 74LCX125 greatly outperform older tech like 74HC125. The performance is faster, the signals have stronger drive, and the drivers have special edge rate control circuits to reduce noise.