SN74HC595: What's the default output after enable but no CLR

Lucas Ding

Part Number: SN74HC595

Hi team,

What's the default output status after enable and give CLK signals but no CLR? My customer feedbacked that it seems the output is uncertain after enable, which means they have to give signals first then enable the output.

Thanks.

over 5 years ago

0 Clemens Ladisch over 5 years ago

Guru 300740 points

The observation is correct. See [FAQ] What is the default output of a latched device? (Flip-Flop, latch, register)

0 Karan Kotadia over 5 years ago in reply to Clemens Ladisch

TI__Genius 16735 points

Hi,

The initial data inside the shift register is unknown. The SRCLR pin will need to be given a low edge to clear out the device. This is possible to achieve with passive components.

https://training.ti.com/tlm-app-gpiooutputexpansion?context=1134826-1139263-1138772

This is a video on how the HC595 works. We will be releasing our new HCS595 very soon and I propose you use this device as it has schmitt trigger inputs and a RC on the CLR pin will clear out the device so that you can have known values on power up inside the latch.

Thanks!

-Karan

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SN74HC595: What's the default output after enable but no CLR