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In my case my vendor and product id were 10c4 and ea60 respectively ( dmesg told me so) usb 1-3.4: New USB device found, idVendor=10c4, idProduct=ea60 CP210x UART Bridge / myAVR mySmartUSB lightĪfter some searching I found advice that recommended a forceful serial mount by writing the vendor and product id to /sys/bus/usb-serial/drivers/ftdi_sio/new_id. When I first try to establish connection to the serial device I was surprised that Ubuntu did not mount the serial device at all (as in there was no /dev/ttyUSB0), even though lsusb lists the device Bus 001 Device 026: ID 10c4:ea60 Cygnal Integrated Products, Inc. Latest JetPack (version 2.2.1) is installed (4th Sep 2016), which should be running the current latest Ubuntu kernel et al.
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Thus, these pins will not source enough current for the MCU to attempt to power itself. If the reverse is true, the CP210x can be held in reset, which will cause all of the UART pins to be open drain and float high with the weak pull-ups. The MCU will effectively see the CP210x pins as open circuits.
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If the MCU is on and the CP210x is off, then the CP210x will not attempt to power itself from any UART lines that are logic high and draw a lot of current from the MCU. In this case, both of the instances you've described below can occur: the MCU is powered when the CP210x is not, or the CP210x is powered when the MCU is off. However, it is not always possible to power both the CP210x and the other device from the same source (if you want to use the microcontroller for something other than USB, for example).
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If both chips are bus powered, we recommend gating the power for the other device with SUSPEND or /SUSPEND, so the device is not drawing power from the bus when the CP210x is not enumerated. In other words, both parts should be powered either from the USB bus or from the "self-powered" configuration. Quote: In most cases, we recommend powering both the CP210x and the device it's interfacing with from the same power source.
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