Level conversion for MCU download

Before talking about MCU download, we need to talk about a few concepts:

TTL level:

The main type of TTL integrated circuit is transistor-transistor logic gate. Most TTL uses 5V power supply. 1. Output high level Uoh and output low level Uol, Uoh ≥ 2.4V, Uol ≤ 0.4V. 2. Input high level and input low level Uih ≥ 2.0V, Uil ≤ 0.8V.

USB signal:

​The USB signal line has two D+ and D-, which are differential signals.

RS232 Level:

On TxD and RxD: logic 1 (MARK) = -3V ~ -15V, logic 0 (SPACE) = +3 ~ +15V.　　

On control lines such as RTS, CTS, DSR, DTR and DCD: valid signal (connected, ON state, positive voltage) = +3V ~ +15V, invalid signal (disconnected, OFF state, negative voltage) = -3V ~ -15V.

start:

max232 converts RS232 signals to TTL signals or vice versa, pl2303 and ch340 convert USB signals to TTL signals or vice versa, so there are two ways to download microcontrollers:

1. USB--->pl2303 or ch340---->TTL--->max232<--->max232---->MCU, there is a line in front (the blue transparent line, which has integrated pl2303 and max232), and the back is on the development board. The level conversion relationship is USB->TTL->RS232<->RS232->TTL.

2. USB--->pl2303 or ch340<---> MCU. The download failed because the MCU was not completely powered off. Solution: connect a diode in series with the TXD pin of CH340 (Schottky diode is recommended) and a resistor of about 300 ohms in series with the RXD pin of CH340. You can refer to the connection method in the STC document. USB->TTL.