STC three-port dual-voltage automatic cold start automatic downloader based on CH340C

1. Features
1. It adopts a USB male plug and can be directly plugged into the USBA port of the computer.
2. It adopts small package and high-level design, making it easy to carry and use. The size is about the same as a USB flash drive.
3. Use pin header, Type-c, Type-A three download port options.
4. Supports 5V voltage and 3.3V voltage output.
5. While realizing automatic cold start, normal communication of the serial port is ensured.
2. PCB
1. Dimensional
board size 42.164mm * 21.590mm * 1.6mm
Actual size 56.896mm * 24.003mm * 12mm
3. Pin
01.VCC
02.RXD
03.TXD
04.GND#
05 Type-c
 
4. Function description
  When the CH340 chip has no download signal, the RST# pin is high level and has no level transition. The monostable oscillation circuit caused by Q1 and Q2 is in a stable state. Q1 is on, Q2 is off, and Q3 is on. Microcontroller power supply.
    When there is a download signal, the RST pin changes level (from high to low), and sends a negative pulse signal to the base of Q1 through the coupling capacitor C13. Q1 is cut off, Q2 is turned on, Q3 is cut off, and the microcontroller is powered off. At the same time, the negative pulse discharges E2, forming a voltage difference across E2. After the pulse ends, E2 is charged, and the circuit is in an unstable state. After a period of time T0, the circuit returns to the steady state, Q1 is turned on, Q2 is turned off, and Q3 is turned on. Restore the power supply to the microcontroller.
T0 is related to the values ​​of R12 and E2, and the calculation formula is:
       ‚ T0 = 0.7*Rt*E2
    In practical applications, it can respond 100%, has high stability and low cost. However, when using serial communication, the circuit may also be triggered incorrectly. Therefore, the RTS# pin of CH340C uses a switch to control whether the RTS# pin is connected to the monostable circuit, thereby selecting the automatic cold start function or the serial communication function.
Because the functional conflict between automatic cold start and serial port communication is resolved. In order to expand the adaptability of this device, a 3.3V voltage is added through 1117. It can also be selected via a switch.
5. Physical demonstration