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Thread: Applying signal on Teensy 4.1 before it is powered up

  1. #1

    Applying signal on Teensy 4.1 before it is powered up

    I am building a prototype that has 5 volt into the Teensy 4.1 supplied from a a separate power source, i.e., not USB. And yes I am cutting the pads on the bottom of the Teensy 4.1 so that the VUSB to Vin is open. My separate supply is connected to the Vin pin on the Teensy 4.1. I don't want to blow up my Teensy 4.1 so I have a two part question to avoid any damage:

    1. Can the Teensy 4.1 tolerate 3.3 volt signals on its IO pins with no power applied to the Teensy 4.1?
    2. If I connect my separate 5 volt supply to Vin (with the cut pads isolating the VUSB), and then I plug in a USB cable into the Teensy 4.1 USB micro connector, do you foresee any issue with the normal development mode starting up and working properly?

    Any insight here would be greatly appreciated.

  2. #2
    Senior Member+ defragster's Avatar
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    Re #1:: Not sure of design statements ... but in practical use the Teensy 4.1 as produced from Beta period was tested with Two T_4.1 having all Serial UART Rx and Tx lines connected.

    When a UART is active it raises the Rx and Tx pins to 3.3V when idle.

    So in that case some dozen+ Rx and Tx wires and GND connected between the two, and each Rx and Tx pin presenting 3.3V.

    Powering either Teensy On/Off out of sync caused no trouble. That is one ON presenting 3.3V on all Rx/Tx pins while the other was OFF caused no trouble while OFF or when then turned ON.

    Re #2:: That seems to describe the normal expected use case. Cutting the VIN<>VUSB pad when given a valid VIN input power source ( with a good GND to that power source to Teensy and each attached device ) will allow connection of a USB cable for programming and Serial USB I/O.

  3. #3
    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by Neal View Post
    1. Can the Teensy 4.1 tolerate 3.3 volt signals on its IO pins with no power applied to the Teensy 4.1?
    No. Pins should not be driven while power is off. While risk of damage is low, it still should not be done.

    The one known problem is with the power management On/Off feature, and also power cycle the board while in "off" mode. If you connect a button to that pin and use it to turn off Teensy 4.1's 3.3V power, having any pins externally driven can cause problems with the power management.


    2. If I connect my separate 5 volt supply to Vin (with the cut pads isolating the VUSB), and then I plug in a USB cable into the Teensy 4.1 USB micro connector, do you foresee any issue with the normal development mode starting up and working properly?
    This should be fine.

    The USB data pins should also not be driven while power is off. But USB is specifically designed for D+ and D- to remain at zero volts before use, so this usage is ok when used with any USB compliant hosts (PCs, Macs, Raspberry Pi, the host port of Teensy 4.1, etc). The host (or hub) has 15K pulldown resistors which keep both pins low. Neither pin gets any signal until the USB device powers up and connects a 1.5K pullup resistor to one of the data signals, so you can be confident the USB host won't drive either data pin until Teensy is powered up.

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