SN74AUP1G32: Output value on startup working as latch.

Ivan Rodriguez

Part Number: SN74AUP1G32
Other Parts Discussed in Thread: TL7702B, SN74AUP1G125,

I would like to implemente a basic latch on this way:

In the simulation, this works properly, but I would like to know if there is any way to change R3 by a pull up resistor to set the output HIGH on startup. When I change R3 by a pull up, the output also starts at LOW level. I assume the output is set to LOW level prior to check the inputs, so the input are also set to low through R2.

Please could you confirm the output startup state?

Thank you a lot!

4 months ago

0 Clemens Ladisch 4 months ago

Guru 253550 points

The initial state is not guaranteed to be low; it is possible that this circuit powers up with a high state.

The only way to force this latch high (or low) is to connect the inputs to VCC (or GND) through a pull-up that is stronger than R2, e.g., 4.7 kΩ.

0 Ivan Rodriguez 4 months ago in reply to Clemens Ladisch

Prodigy 20 points

But if I place the stronger pullups, it would not work as latch anymore because the feedback resistor is weaker. Is there any other solution to setup the initial state on this device?

Thank you a lot!

0 Clemens Ladisch 4 months ago in reply to Ivan Rodriguez

Guru 253550 points

That stronger resistor would need to be switched.

Do you have a reset signal? If not, it could be created with a R-C circuit or a voltage supervisor IC.

0 Ivan Rodriguez 4 months ago in reply to Clemens Ladisch

Prodigy 20 points

Could you attach an schema with your solution?

Thank you a lot!

0 Clemens Ladisch 4 months ago in reply to Ivan Rodriguez

Guru 253550 points

You would connect the active-high output of the TL7702B to the inputs.

But such a reset chip is quite expensive. What are your requirements regarding cost, space, and power usage?

0 Ivan Rodriguez 4 months ago in reply to Clemens Ladisch

Prodigy 20 points

I was interested in the RC circuit option. Power usage is a big limitation, because it is battery powered. This circuit would be always on.

Thank you!

+1 Clemens Ladisch 4 months ago in reply to Ivan Rodriguez

Guru 253550 points

Connect the R-C circuit to the /OE input of a three-state buffer like the SN74AUP1G125; the input is always 1. The buffer will pull its output high only while the /OE input is low; then it will be inactive.

0 Ivan Rodriguez 4 months ago in reply to Clemens Ladisch

Prodigy 20 points

Okey, with the three-state buffer is ok, I thought it would be possible with the SN74AUP1G32 and I didn`t come with how.

Thank you a lot.

0 Clemens Ladisch 4 months ago in reply to Ivan Rodriguez

Guru 253550 points

… and you don't need an OR gate for the latch; any buffer like the SN74AUP1G125 would work.

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SN74AUP1G32: Output value on startup working as latch.