SN74HC374: SN74HC374PWR input floating

utsav kumar1

Part Number: SN74HC374

For one of my design I am using SN74HC374PWR latch, and this latch is getting driven by micro controller.

As the input of latch is directly driven by micro controller and there is no pull up/down resistor connected, will there be any problem to the latch IC when micro controller is getting power on reset? I mean will this damage the latch IC or hang the latch IC?

By default pull up resistor is connected to the /OE and pull down resistor is connected to CLK pin and as per datasheet, to change the status of output, /OE has to be pulled low and clock should have rising edge.

But, while power cycle when /OE is pulled high and CLK is pulled low, will there be any risk on the latch IC as the input of latch is connected to microcontroller only (due to undefined state of microcontroller while power cycle) and not connected to valid logic (pulled up/down)? Please refer the snapshot in the previous email.

over 4 years ago

0 Clemens Ladisch over 4 years ago

Guru 263000 points

CMOS inputs must no be left floating; see [FAQ] How does a slow or floating input affect a CMOS device?

However, the time between the latch being powered up and the microcontroller configuring its GPIOs should be short enough that no harmful charge can build up. (In the example shown in this thread, open inputs float quite slowly.)

If you are paranoid, add weak pull-up or pull-down resistors to the D inputs.

0 utsav kumar1 over 4 years ago in reply to Clemens Ladisch

Prodigy 20 points

Hi,

Thanks for your reply!

Just to highlight, my board is not having enough space to accommodate 8 new resistors easily, if it is not really required then i would like to avoid adding them.

To give you more information, in my design the time taken by micro controller to come out from reset and configuring its GPIOs to output mode & driving them low is <10mS. And also, most of the time my board will be ON and its most rare that board will see power ON reset.

Having this information in mind, do I still need to add weak pull-up or pull-down resistors to the D inputs?

Thanks,

Utsav Kumar

0 Dylan Hubbard over 4 years ago in reply to utsav kumar1

TI__Mastermind 43690 points

Hey Utsav,

I would say the risk is definitely low in this case and you can probably get by without them.

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SN74HC374: SN74HC374PWR input floating