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I wonder if I need pull-up resistors when using the PCF8575 I/O extender.

It's not directly mentioned in the datasheet (http://www.ti.com/lit/ds/symlink/pcf8575.pdf). The datasheet does remark that the I/O ports are quasi-bidirectional push-pull design, but I'm not sure if that says anything about internal pull-ups.

The PCF8575 is connected to ACPL-247 opto-couplers for input (collector/emittor side) and NCV8402D mosfets for output.

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  • \$\begingroup\$ Surely whether it needs push-pulls is at least partly dependent on your design? If they're used as outputs they won't need them. If they're used as inputs, it depends what they're connected to. \$\endgroup\$
    – DiBosco
    Sep 8, 2017 at 14:35
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    \$\begingroup\$ "I'd add them just to be sure" To be sure of what? \$\endgroup\$
    – m.Alin
    Sep 8, 2017 at 14:35
  • \$\begingroup\$ @DiBosco: MOSFET gate pull up for outputs. Input side is connected to an opto-coupler transistor, pull up would go on the collector leg to the PCF port. \$\endgroup\$
    – svenema
    Sep 8, 2017 at 14:40
  • \$\begingroup\$ @m.Alin: As in better safe than sorry.. But that is more like a breadboard approach. When designing a PCB I don't like second guessing this.. \$\endgroup\$
    – svenema
    Sep 8, 2017 at 14:41
  • \$\begingroup\$ Open collector opto? \$\endgroup\$
    – DiBosco
    Sep 8, 2017 at 14:42

2 Answers 2

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The only real need for pull-ups would be to ensure that either the output starts or goes high when the output is not driven or to force the pin into a particular state when the pin has an open input.

The spec sheet says "At power on, the I/Os are high. In this mode, only a current source (IOH) to VCC is active." That is, the maker already thought of that for you, assuming you don't need it to start out low....

However, if your particular design needs to hold a particular pin in some state while you switch it around, that is another matter.

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  • \$\begingroup\$ Start position is not really important. I can set these over i2c at start up. \$\endgroup\$
    – svenema
    Sep 8, 2017 at 14:47
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    \$\begingroup\$ @svenema well assuming one pin doesn't turn on the 'Launch ICBM" N-Channel MOSFET when high... yes. \$\endgroup\$
    – Trevor_G
    Sep 8, 2017 at 14:49
  • \$\begingroup\$ But, in the case of an input. The port is connected to an opto-coupler transistor, this probably means that the port will float when the input is not driving the led in the opto-coupler. So I may need a pull-up for that situation? \$\endgroup\$
    – svenema
    Sep 8, 2017 at 14:50
  • \$\begingroup\$ If the opto is pulling the pin to ground you should be ok assuming you initialize properly first. \$\endgroup\$
    – Trevor_G
    Sep 8, 2017 at 14:55
  • \$\begingroup\$ I think an photo-transistor is not conducting whenever the LED is off, so I think it's not pulling anything to the ground. It's when the internal LED turns on that it pulls to ground, given that the emitter is connected to ground. \$\endgroup\$
    – svenema
    Sep 8, 2017 at 14:59
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You don't need additional pull-up resistors.

The PCF8575's datasheet says that:

At power on, the I/Os are high.

So the outputs are in a known state.

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