You probably already know this because you want to use the USB-UART for power which makes it very convenient and also is likely to cause fewer problems with many small device setups than anything else..
I find the CP-2102's power adequate for many small devices..
But for the benefit of others reading - its always good to remember, if the other thing you are plugging into has its own power, make sure the common ground is solid, and then only use three wires.. TxD Rxd and that Gnd.
Unless you are absolutely sure it wont cause problems- especially if one of the wires- like the ground is momentarily lost - if your ground is solid, that is quite helpful- Its also helpful if there is only one PS -
don't have multiple power supplies feeding into something.. if you can use the same USB for power thats quite helpful. Alternatively, use separate supplies but have a strong common ground that wont accidentally come undone and then just use the UART lines for the signal and not power
Or you could conceivably fry your device or even your computer.
Yes indeed. And it turns out that this issue of which power source should be used can be tricky. For example, if you power up the device being flashed from its own supply, and limit the USB adapter's connections to TX, RX and GND, you still have the issue of what voltage the device's Vcc is set to. if it's 3.3V or 3.6V, then that should be no problem. But if you're running a low-power MCU at 2V, then you're going to be feeding TX to it at 3.3V, because that's what the adapter outputs, and that may violate the MCU's absolute rating for voltage applied to any pin relative to Vcc, and smoke may escape.
There can be a similar issue related to which device gets powered up first. If you power up the adapter via USB before you power up the device, then you're applying 3.3V from TXD to a processor whose Vcc is still at ground. Some processors are designed to deal with that well, but not all.
So I would agree that in general it's less likely to cause problems if you can power the device from the adapter's regulated 3.3V output, and leave its normal supply switched off. Everything comes up at the same time, and the voltages are always right. But if you can't do that, then you need to think through exactly how this is going to happen, and check the datasheet for those absolute ratings for the device you're flashing.