Please find my comments for your questions below:
1. The MIPI_RESET should be pulled down to ground using 10K resistor (as mentioned in the datasheet).
The CX3 chip will internally drive this pin high during power up. You don't need to connect any external component for the same.
2. From the datasheet, VUSB should be within 4-6V. As VUSB drops down to 2.5V and stabilizes only after all other power domains stabilize, this can be a cause for concern. Please let me know if the device is enumerating properly. Also, share the schematic so that I can evaluate it on my end.
Thanks for your reply.
Regarding question #2, VUSB is the input voltage to a PMIC that generates the other voltages for the CX3 (AVDD, VDD, VDD_MIPI, VDDIOx, and CVDDQ). As such, I can change the UV/LO input threshold on the PMIC such that the other voltages don't start to enable until after VUSB reaches ~3.5V. After this, VUSB continues to rise monotonically until is stabilizes, and so it won't dip below 4V whilst the other votlages are coming up. Unless you foresee any issues, I think I will take this approach.
In response to your question, we have not seen problems with enumerating on a consistent basis. However, we have some units that behave sporadically, so I think this issue might be a factor. I'm not able to send the schematic at this time because technically it belongs to our customer (we are the ODM/OEM) but should it become necessary to distribute I can seek their permission.
Can you please share the current waveform on VUSB.
Also, the workaround you mentioned in your previous response should be fine taking into consideration that the Power up sequence mentioned in fig 4 of the CX3 datasheet is not violated.