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[FAQ] TPS546D24A: Does the USB-to-GPIO dongle have internal pull-ups? What resistance? To what voltage?

Part Number: TPS546D24A

The TI USB-to-GPIO interface dongle does include internal pull-ups to the on-board 3.3V supply voltage.

The Pull-up resistor values are selectable with 4 levels, set through the USB Adapter Settings option under the TOOLS menu in the FUSION GUI

The options are:

Open Drain (No internal pull-up)

2.2kOhms

1.0kOhms

688-ohms (2.2kOhms parallel with 1.0kOhms)

What if I want to be able to use the TI USB-to-GPIO interface dongle on my board, but my parts can't handle a 3.3V bus voltage?

There are a few options available.

1) The simplest option is to place lower voltage pull-up resistors on your board and select the "Open Drain" pull-up resistor option in the USB to disable the adaptor's internal pull-ups.

2) If the Adaptor will need to drive the bus when the on-board termination voltage is not powered for some reason, you can add zener clamping diodes to the CLK, DAT and ALERT pins to limit them to a voltage safe for all devices connected to them, allowing the adaptors integrated pull-ups to terminate the bus when the on-board termination voltage is not available.

Make sure you include the clamping diode's junction capacitance in the calculation for the total bus capacitance.

3) It is also possible to scale the termination resistors so that the bus voltages will not exceed the safe rating of the connected devices even when both termination voltages are available by scaling the termination resistors.

For example, with a 2.2kOhm pull-up to the internal 3.3V supply and a 301-ohm pull-up to an on-board 1.8V supply, the maximum CLK and DAT voltage would be 1.98V, meeting a 1.8V +/- 10% tolerance with

Note:  Pull-up resistor selection, along with PEC and Switching Frequency settings are stored in NVM onboard the USB-to-GPIO adaptor and will remain active with the adaptor until they are changed again, even if the Adaptor is disconnected and unpowered.