Can someone tell me what does latch on power up mean?

I have a deserializer which has latch on power up pins.

Like, from what I understand, during the power up of the Deserializer, those latch on power up pins, will be inputs and take in High or Low values. Once powered up, those pins will be configured as outputs. Is my understanding correct?

If not, please let me know.

If it's correct, please tell me how does the latch on power pins take in high or low values DURING power up? Like, the power is not even stable yet, but how can you read the value?

I am unable to share the datasheet since it is under NDA.

  • \$\begingroup\$ Why did you capitalize "during"? What aspect do you not understand? \$\endgroup\$
    – CL.
    Jun 1, 2021 at 6:43
  • \$\begingroup\$ I didn't understand like, how can a pin read HIGH or LOW, During Power up. Like, the power in not even stable yet, but how can you read the value? I am not sure on how this latch on power pins work. Can you provide a detailed answer on why such pins are required and how does it work? Like, how it reads the input DURING power up? \$\endgroup\$
    – user220456
    Jun 1, 2021 at 6:51
  • 1
    \$\begingroup\$ Your understanding is correct. Re your second question, note that Vcc/2 is Vcc/2 whatever the value of Vcc is. Deciding whether a pin is above Vcc/2 or below is relatively easy. \$\endgroup\$
    – user16324
    Jun 1, 2021 at 13:28

1 Answer 1


Here, "power up" does not only mean the time during which the power supply voltage is rising; it also means some time afterwards during which the device is initialized.

After the voltage has become stable (or at least high enough that the logic gates work reliably), the voltage at the pins is measured, and then the pin is configured as output. Only then does the device begin to do its actual work.

(This is exactly the same how you would implement it when you were to program a microcontroller: your initialization code runs after the power has become stable.)

  • \$\begingroup\$ Thank you for the answer. Just one final question which I forgot to include in my question. Does the pin automatically change to output after the voltage at the pins are measured once powered up? \$\endgroup\$
    – user220456
    Jun 1, 2021 at 7:03
  • 2
    \$\begingroup\$ Since we don't know which chip and datasheet is under NDA, how can we know if those pins become output automatically, or via some manual operation, or if they even become outputs at all? If you don't understand the datasheet, ask the chip manufacturer because you seem to have their attention if you have an NDA. \$\endgroup\$
    – Justme
    Jun 1, 2021 at 7:14

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.