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Camera hangup - power on reset problem

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  • Camera hangup - power on reset problem

    See Fig1. It is typical RS422 to 3V3 level shifter. Camera module have separate power suply and is placed aprox. 10 meters from receiving device. There is no problem on first power-up of this device. Problems starts when remote camera is switched off and on.
    Off situation is shown on fig.2. Due pull-up (R1) supply voltage do not falls to zero, but remains around 1V. It is well outside of operating conditions of camera module and you are traped.
    There is no voltage monitoring / reset IC on camera module and no way to assert external reset via some pin.
    Similar problem can arise with some RS232 -> 3V convertors. It is not uncommon situation to have unpowered device connected to your PC.

    In case that you have sync problems please check:
    "Supply voltage" of camera module in unpowered state with connected level shifter
    Put weak pull up resistor to camera Rx pin in case that camera boot is faster than your MCU uart initialization.

    Only dirty solution, for my case, was put balast power drain resistor between 3.3V and 0V to minimize R1 effect at the cost of higher supply current.

    If there will be Cam TTL version 2, please put in place some voltage monitor or make reset avaiable on pin.

    Another solution for HW reset: use com break condition, If RX line is held low for more than 100 ms(?) -> Device reset.

    Have a nice Day
    Daniel

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