• State Resolved
  • Locked Locked
  • Replies 1 reply
  • Subscribers 33 subscribers
  • Views 135 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

PCF8574A: Leaving 3V3 on input pins while removing VCC for resetting the IC

RK
Prodigy 50 points
Part Number: PCF8574A
Other Parts Discussed in Thread: TCAL9539

Tool/software:

Hello,

we are considering using the PCF8574A Remote 8-Bit I/O Expander and reading the datasheet following questions came up:

Is it allowed to have 3V3 on the P[0..7] IO-Pins while the supply voltage is removed from the VCC pin?

The absolute maximum ratings for VI say:

VI Input voltage range: –0.5 to VCC+0.5

Does this mean that having 3V3 on P[0..7] while VCC is 0V would exceed the maximum rating or is the maximum for VI always 6.5+0.5V=7V no matter if VCC is applied or not?

Background is that we want a way to reset the PCF857A by removing VCC but in this case the input voltages on P[0..7] would still stay at 3V3 at the time VCC is removed.

Thank you and best ragards

RK

  • +1
    TI__Mastermind 46703 points

    RK,

    This device doesn't have powered-off high impedance on the I/O pins. 

    There is a diode from P0-P7 to VCC. Thus there will be back-powering into VCC when VCC = 0V and P0-P7 = 3.3V. 

    This diode is removed in other I/O expander designs such as TCAL9539 for example. TCAL9539 removes the pin to VCC diode. The voltage on the I/O can be above the supply voltage to a maximum of 3.6V. 

    Regards,

    Tyler