[General] 74HC595 Default LED Flash

[elec_gyan]

Hello All,
Though this question might seem old, but I haven't found relevant answers to the problem I am facing.
In the attached circuit, I observe that all LEDs flash for around 120ms (measured) during power up.
I have tried below options but still the problem seems to be unsolved:

1) Connecting OE* directly to ground
2) Pull-down the signals going from microcontroller to 74HC595 as the microcontroller pins are in floating state during RESET.
3) Connecting MR* to microcontroller RESET so that it remains LOW till microcontroller is out of RESET.

Is it because that 74HC595 is acting as sink that might be causing some leakage current to activate the LEDs?

Kindly provide your suggestions.

 

[KlausST]

Hi,

I assume all internal registers are LOW after power up.
Now if the outputs are enabled (OE_N = LOW) before you shifted valid data into the HC5959 all LEDs will be ON.

It seems your microcontroller takes a lot of time (120ms?) after RESET before it sends data to the HC595.

****
My recommendation:
Connect a pullup on OE_n

This ensures high impedance (LEDs OFF) after power up.

Then serially shift all data into the HC595.
After finished data transmission: pull OE_n LOW. Maybe with the use of an additional port pin.

***
An RC delay may also work.
C: one leg to VCC, other leg to OE_n.
R: one leg to GND, other leg to OE_n.
R*C time constant should be more than the 120ms. (depends on V_IL of OE_N pin)

Power OFF for sufficient time for the C to discharge completely. Or use a diode for faster discharge.

Klaus

 

[elec_gyan]

Hi KlausST,
Thanks for suggestions, yes I measured that the microcontoller takes 330ms to come out of RESET. This is because the microcontroller needs ACTIVE LOW and I am using a NAND gate between MAX810 & STM8S to do so along with RC filter. Connecting the RESET pin with 10k pull-up solved the problem. This ensure the microcontroller is immediately out of RESET after power up. I am no more using the NAND gate now.

Anyhow, I will try the RC option suggested by you.