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PCF8575: Use PCF8575 as input

YixiangYang
Prodigy 10 points
Part Number: PCF8575

Dears,

Thanks for your support in advance.

1. Below is the Block Diagram of PCF8575 from datasheet( I marked 1,2,3 three MOSFETS,), in my understanding this P-port is push-pull output (MOSFET2 and MOSFET3),

my question is: when configure PCF8575 as input port, how the MOSFET works?

2. Below is the schematic of my application, PIN10-11，PIN1-8 were used, PIN12-PIN17 were not used

my question is: If all P-ports are used as input, whether PIN12-PIN17 can be left floating, or should be connected to GND（directly or by a resistor(what the value should be)）?

3. Below is the I2C waveform of my circuit, byte1 is:01000011,byte2 is:01111111, byte3 is11111111 (for byte1 is from Master, and every time master send 01000011, PCF8575 will send back PIN1-8,PIN10-17 condition, )

my question is: Whether byte1:01000011 can configure PCF8575 P-port as input port?

  • 0
    Guru 280180 points

    The PCF has quasi-bidirectional I/Os. This means that they can strongly drive low, but only weakly pull high. The direction cannot be configured directly; to use a pin as input, configure it to output high (which is the default).

    1. MOSFET 1 strongly drives high when the register is written. This happens only for a short time. MOSFET 2 weakly drives high (the current source actually is 100 µA). MOSFET 3 drives low.

    2. Unused pins can be left open; the internal pull-up prevents them from floating.

    3. The first byte contains the device address, and the R/W bit. When reading, the PCF always returns all 16 bits.

  • 0
    TI__Mastermind 31513 points

    Hi Yixiang,

    YixiangYang said:

    1. Below is the Block Diagram of PCF8575 from datasheet( I marked 1,2,3 three MOSFETS,), in my understanding this P-port is push-pull output (MOSFET2 and MOSFET3),

    my question is: when configure PCF8575 as input port, how the MOSFET works?

    Your understanding is correct. P-port is push-pull configuration. Upon initial power up of the device, I/O's are defaulted HIGH by current source 100uA current source (says 100mA in datasheet, this is an error). 

    MOSFET 1: Used as a strong pull-up to VCC when an output is written HIGH and is switched off by the negative edge of SCL. 

    MOSFET 2: Weakly drives HIGH through 100uA current source to VCC. 

    MOSFET 3: strongly drives LOW

    YixiangYang said:

    2. Below is the schematic of my application, PIN10-11，PIN1-8 were used, PIN12-PIN17 were not used

    my question is: If all P-ports are used as input, whether PIN12-PIN17 can be left floating, or should be connected to GND（directly or by a resistor(what the value should be)）?

    Unused pins can be left open since internal 100uA current source pulling up to VCC is present. 

    YixiangYang said:
    3. Below is the I2C waveform of my circuit, byte1 is:01000011,byte2 is:01111111, byte3 is11111111 (for byte1 is from Master, and every time master send 01000011, PCF8575 will send back PIN1-8,PIN10-17 condition, )
    YixiangYang said:
    my question is: Whether byte1:01000011 can configure PCF8575 P-port as input port?

    P-port are initially configured as input upon power up. Write to device register using 0x66 (01000010) to configure P-ports as input or output. 

    Regards,

    Tyler