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.

PCF8574: Difference PCF8574 vs PCF8574A, high-side FET

Part Number: PCF8574

Hi,

I have two questions about the PCF8574 device:

1.
What are the electrical differences between the PCF8574 and the PCF8574A (beside the address range)?
I'm aware of the document scyb031. In one E2E thread I have read that for new designs it is better to use the PCF8574A. So I assume there are electrical improvements (?).


2.
We use 4 of the Port pins as inputs (P0 to P3) and the other 4 as outputs (P4 to P7). On the inputs we have 4 switch connected to GND. After power-up all the port pins act as inputs respective outputs with a weak high-side current source (all the bits have a logic 1).
After power-up I need to switch the 4 outputs into a logic 0 state. Therefore I have to write 0x0F to the I/O expander. As of my understandig of the datasheet, while I'm doing that, the lower 4 port pins will activate the second high-side FET for a short period of time (IOHT, in figure 16). Is this current limited to a certain value? All the 4 switch at the P0 to P3 can be closed at this time and therefore the port pins are connected directly to GND. Or does this second high-side FET gets activated only if a port pin has to change from a logic 0 state to a logic 1 state?

Thanks and best regards,

Patrick



  • Hi Pruf,

    1.What are the electrical differences between the PCF8574 and the PCF8574A (beside the address range)?

    I don't believe there are any electrical differences between these devices apart from the addresses. We likely recommend using PCF8574A for newer designs for production and logistical reasons, not electrical characteristics. 

    2a. Is this [Ioht] current limited to a certain value? 

    I believe this current is limited, but I cannot find a value associated with it. The datasheet describes this current value to be -1mA typical while the pin is shorted to ground. In your application, this will likely be the case if the input switch is connected to ground. If this is a concern, you may consider adding a small series resistance to further limit the current during this short pull-up period. 

    2b. Or does this second high-side FET gets activated only if a port pin has to change from a logic 0 state to a logic 1 state?

    I also could not find a definitive answer for this. Let me know if this will be an issue given the above information. 

    Note TI offers other IO expanders with configuration registers to allow input ports to be high-impedance. This may simplify the hardware design with minimal changes need to software. Let me know your system requirements if you would like device recommendations from this category.

    Regards,
    Eric

  • Hi Eric,

    2a. This is not a concern as long as the device (PCF8574) does not get damaged (the switch can handle higher currents). I also believe that this current (IOHT) must be limited as otherwise it would not be possible to have inputs and outputs mixed on one device.
    Or the second high-side FET is only activated when a port has to change from a logic 0 state to a logic 1 state (bot activated when a port is already in a logic 1 state)

    A bit more information to this specific topic in the datasheet would help. Anyway thanks for your answers.

    Best regards,

    Patrick