U1 is CH376S chip; X1 is 12MHz; C4 is for decoupling to reduce EMI during USB transmission capacity is from 4700pF to 0.1uF; P4 is USB port; R1 is for limiting external USB current; P5 is SD card socket suitable for standard SD card; R3 is for limiting current of external SD card; P1 is 8-bit signal port signal includes D0-D7 A0 RD# WR# CS# and GND while INT# is optional; P2 is SPI signal port of series port SPI signal includes SCS SCK SDI SDO and GND while INT# is optional; P3 is signal port of asynchoronous serial signal includes RXD TXD and GND while INT# is optional;J3 is for choosing CH376 chip; Working voltage: 1-2pin: 5V 2-3pin: 3.3V