SN74LVC1G373: Trouble with status of Q output after power-up

sebastien salmon

Prodigy 30 points

Part Number: SN74LVC1G373
Other Parts Discussed in Thread: SN74LVC1G175, TLV809E, SN74LVC1G98

Hello,

Here is my set-up before powering up :

- Vcc and D input are tied together (in 3.3V level)

- LE signal cames from another part (in 3.3V level) and is set-up to 0 (gnd)

- /OE is tied to ground

When i put the power on (Vcc going from 0 to 3.3V), the Q output switch to 1 but i was expecting this output to remain at 0 as LE signal is still at 0. But this is not the case in reality.

Here a view of the electronic set up.

Thank you for helping me to solve this problem

over 4 years ago

0 Clemens Ladisch over 4 years ago

Guru 271250 points

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

Apparently, you want a circuit that powers up with the output at 0, and permanently switches to 1 at the first rising edge.

This can be done with a flip-flop like the SN74LVC1G175. To initialize it to 0, you have to create a low pulse at power on for CLR, either with a reset IC like the TLV809E‍C26, or a resistor+capacitor.

Alternatively, construct a set/reset latch from two NOR gates (e.g., SN74LVC1G98); it also needs to be initialized (with a high pulse).

0 sebastien salmon over 4 years ago in reply to Clemens Ladisch

Prodigy 30 points

ok I understand better now, and i will set-up this (sn74lvc1g175) for a next revision of my board.

But as PCB is manufactured, is there any possibility to have it working by maybe using a pull-down on the Q output ?

Thanks you a lot

0 Clemens Ladisch over 4 years ago in reply to sebastien salmon

Guru 271250 points

Q is an active output; it would override the pull-down resistor. You would need some mechanism to keep OE high during that time.

To make the Q output drive low, you need to have D low and LE high. There is no easy way to do this with this board.

0 sebastien salmon over 4 years ago in reply to Clemens Ladisch

Prodigy 30 points

Thank you a lot, I'll try the part you advise me !

Logic

Logic forum

SN74LVC1G373: Trouble with status of Q output after power-up