This is a program verification board. The main verification content is the function of USB to RS232 signal. One end is a USB plug and the other end is a DB9 female connector, which can be directly plugged in.

a.    USB to 232 can be converted directly by a special chip, but here is the conversion between USB and TTL through a CH340N, and then a MAX232 chip for conversion between TTL and 232.

b. In order to avoid waste of functions, the functions of the two are brought out through a row of pads:

1.   The TXD and RXD pins of CH340 are led out, and a ground is added to realize the conversion between USB and TTL, which can be used to realize serial communication, such as the serial communication of a computer and a single-chip computer.

2. The other 232 and TTL of MAX232 are also led out, because MAX232 contains two conversions. Therefore, the first way can be converted through the plug and RXD and TXD pins, and the other 232 and TTL levels can be realized by several pads marked as 232-T2, 232-R2, TTL-T, TTL-R and ground on the silk screen. Conversion

c. There is also the problem of power supply of the chip. Since both chips can be used in 3.3, the power supply of the two chips is set to the circuit when the power supply is 3.3V. The source of 3.3V is mainly an LDO conversion circuit. When plugged into the computer serial port, the 5V of the computer serial port can be converted to 3.3V for use by two chips. In addition, the 5V and 3.3V are also led out, so external power supply can also be used. It is not recommended to use the LDO on the board to generate The 3.3V external power supply.