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PCF8575 problem with output drive

GB Rossi10
Prodigy 20 points
Other Parts Discussed in Thread: PCF8575, PCF8575C

Good morning

I have a problem driving output of PCF8575 . I have all output disconnected (they are floating)

now on I2C I only send this command

START  I2C

0x40 ---   write at address 0x40    ------>  ACK 0 level from PCF on ninth bit


oxFF  ---  all output on      ----->>   ACK ACK 0 level from PCF  on ninth bit


oxFF  ---  all output on      ----->>   ACK ACK 0 level from PCF  on ninth bit

STOP I2C

All outputs at last ACK go at level 1 3,3V  but after STOP condition they return to 0V   like a discharging capacitor

Why this behaviour?

must I send a 0x00 command before send write command?

Thank you very much

  • 0
    TI__Guru* 96591 points

    Hi,

    It looks to me like you are sending the correct commands, but I'm curious as to exactly what happens when 'STOP I2C' is indicated.  Do you happen to have a logic analyzer connected to this circuit to show exactly what is happening there?

    If not, can you share the code being used?

  • 0 in reply to Emrys Maier
    Prodigy 20 points
    thank you very much for your answer.
    The problem is the following: I'm using PCF8575C instead of PCF8575 ... so there isn't the circuitry for high level maintenance. there are two different datasheets for these devices and I purchased the wrong one. Now with PCF8575C I have to pullup all pins externally, correct?
  • 0 in reply to GB Rossi10
    TI__Guru* 96591 points
    You are correct - the benefit for this is that you can control how strong the pull-up is with PCF8575C, whereas PCF8575 always has a 100uA internal pullup.