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PGA308-Q1: 1W interface clarification

David Shumate
Expert 7535 points
Part Number: PGA308-Q1
Other Parts Discussed in Thread: PGA308EVM, PGA308, PGA309

Tool/software:

Hi Team,

My customer is looking at the PGA308-Q for a strain gauge.

I’m not very familiar with the 1W interface. I read through the datasheet and manual and I see the timing. Do you have any extra materials on this interface? It looks like we can use the MCU UART with a pullup resistor for this interface, but I’d like to double check. Can you confirm?

Thanks,

David

  • 0
    TI__Mastermind 23738 points

    David,

    You are correct.  Any MCU UART should work well for the one-wire interface.  In the case of the PGA308EVM, the 1W signal is generated by the USB-DAQ-PLATFORM.  The USB-DAQ-PLATFORM,  ( USB-DAQ-PLATFORM Schematic ) uses a "one-wire speedup circuit" to reduce the rise and fall time of the digital 1W signals.  It does this by using an FET drive to increase the drive current.  This is especially important when driving long cables (i.e. high capacitance).  The PGA309 is a similar device that also uses a one-wire interface.  You may want to look through the literature for PGA309 for additional information on the one wire interface.  If you have any specific questions regarding the interface let me know.  Many engineers use the USB-DAQ-Platform to program their PGA308/309 devices.  However, you can develop your own interface with most microcontrollers.

    Here is the firmware for the USB-DAQ-PLATFORM:  USB-DAQ-FIRMWARE.  USB.c has the OneWire(void) function that performs the 1W communications.  This function is activated from our EVM software by sending commands, and listening for responses.  I'm not sure if the firmware will help you but it is what the EVM uses to implement communications.  

    Best regards, Art

  • 0 in reply to Art Kay
    TI__Expert 7535 points

    Thanks Very much Art! Could you also comment if we have any recommendations for the UART interface (eg pullup resistor value/location)

  • 0 in reply to David Shumate
    TI__Mastermind 23738 points

    David,

    If your cable between the USB-DAQ and PGA308  is a short cable, then you don't really need the one-wire speedup circuit.  This is especially true if they are on a PCB together.  The speedup circuit is mainly for driving the capacitance of a long cable.  Sometimes people program the device through a cable more than 3 meters long.  If you don't use the speedup circuit, I think a 4.7k ohm pull-up is typical.  You should probably consult with your microcontroller data sheet but I think you could use a range of pull-ups.  If you have an evaluation PCB for your microcontroller, you might consider connecting it to the PGA308EVM test board to be confidant in your communication approach.  

    Best regards, Art