Power surge with 74HC595

alnajjar1

Hello,

I am using a BS2 and 74HC595 in a chain to turn on 64 LEDs. It works beautifully but the only problem is that upon power up, all LEDs come on for a fraction of a second of a second. I send a "0" to all the 595s in the first line of the code but still have the initial All-on at start up. How can I avoid that?

Many thanks in advance

Al

brucee

Connect up OE and control it from the BS2. Drive it low only after you have loaded the shift register.

Even then if it is floating you may get a flash on power up, and to avoid that connect a pullup resistor to it, 10K or 100K would be OK.

alnajjar1

Thanks. To make sure I understand, you are suggesting connecting the OE line to a BS2 pin and pull it low to enable the chip after registers are loaded and the 10k pull up is to make sure OE is not floating, correct? I am attaching a schematic with your suggestion to make sure I am on the right track.

Since I have 8 of these 595s in a chain (as you see in the schematic), do I need to do the same for every IC or only the first in the chain?

Many thanks again,

brucee

You can drive all the OEs together, much as you are driving the CLK and LATCH signals. You don't need the 330 ohm resistor.

alnajjar1

Sounds great, thanks.