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In our custom implementation of the FX3 chip, we are using it as a device. We have VBATT connected to 3.3V through a 0 ohm resistor, and the VBUS pin connected to the voltage line of the USB connector through an overvoltage protection in a similar manner to the dev board. The documentation states that when both are connected, the VBUS line is used by defatult.
My first question is, if the VBUS line loses power, will the chip switch to the VBATT for power?
Second question is, what happens to the internal operation of the chip if VBUS is power cycled? Is anything reset? We have all other I/O and core voltages powered internally.
Yes, when there is not voltage on VBUS,FX3 will use VBAT supply for power
When VBUS is power cycled, the FX3 device has the ability to detect the presence of VBUS (this can be controlled using CyU3PUsbControlVBusDetect API). So, this will help to identify whether the device is connected to the PC. Although, with the VBUS goes of, the FX3 device will be active by self-power VBAT. Once it detects a VBUS, then it can send the descriptor to the host to enumerate again.