Is there a small (WLCSP, <=3mm*3mm) USB MCU that has good library support (USB UART w/flow control) and is available in actually manufacturable package (pitch>=0.5mm)?
I came across SAML22, but its 0.4mm package is a huge PITA. PSoC5LP is a good option, but it's way too big, and way to over powered and expensive.
I don't need a ton of pins. Something like EFU8UB10F16-QFN16 is perfect, and it also integrates a voltage regulator, which is much needed, only if it's in WLCSP package.
I'm open to any MCU architecture -- be it 8051, AVR, PIC, MSP430 or any ARM. If he library and compiler is good, I'm even open to 4-bitters.
Any ideas?