I'm planning on getting tiny USB serial SIP modules made that use the same USB chip as the P2D2 but here are some of the features of the P2USB chip which btw are up to a 1/4 of the cost of the much larger (and dumb) FT232R.
* Silabs EFM8UB3 40k Flash, 48MHz internal - small 3x3mm QFN
* Up to 8Mbd operation
* Disconnects from P2 RXD when not enumerated by host PC
* No phantom power effects - always on
* Holds P2 in reset even if Vin is below operating thresholds, until power is good
* Only resets P2 on valid serial loader pulse - rejects DTR toggles etc
* Forces P2 boot to ignore Flash and SD on serial load or long button press
* Bidirectional external reset request signal - reset external chips.
* SD power control - always pulses SD power off for a clean reset during reset or by I2C command
* Watchdog control via I2C - defaults to: off at power-up, once triggered can only be retriggered by 8-bit count (5ms .. 1.275s)
* Loads Si5351 clock generator during reset with standard 25MHz or 26MHz crystal for P2 20MHz or user clock configuration.
* I2C commands read analog voltage reference, 1.8V supply, 3.3 rail, USB, and P63 to P56 analog levels
* Monitors P2 transmit data for special escape commands
* 128-bit silicon UUID code
* User read/write of USB config tables in Flash
* 16kB Flash EEPROM emulation - user data independent of SPI Flash and SD
* Timing capture of P2 signals - identify P2 clock rate inc. RCSLOW/FAST
* P2 SPI and I2C port monitoring and control.
* Access P2USB internal registers and memory etc over I2C
* Firmware update over USB as HID