MCU download program circuit principle USB to serial port line, CH340, PL2303, MAX2

        The first step in microcontroller software development is to download the program. If there is a problem with this step, then everything else will be out of the question. Today, I will give a detailed explanation of the problem of downloading programs for commonly used 51 microcontrollers. If there are any mistakes, I hope you can point them out!

Principle: TXD and RXD of the microcontroller are TTL level, so you have to convert other signals into TTL level. Only in this way can you download the program to the microcontroller successfully! Among them, chips such as CH340 and PL2303 directly convert USB signals into TTL level, while chips such as MAX232 convert TTL to RS232 signals or RS232 signals to TTL. Please see the following two common methods using this principle:

method one:       

        Please see Figure 1, which is a common single-chip computer download circuit. Among them, from ② to ⑥ are commonly used USB to serial port cables. The advantage of using this solution is that if your PC has a serial port (even very old machines do not have USB interfaces), you can directly download programs to the single-chip computer development board, because the development board using this method must have a serial port interface. Of course, if there is no serial port, you can only find a USB to serial port cable (the circuit principle is ② to ⑥ in the figure). It is recommended to use the USB to serial port cable of the CH340 chip instead of the USB to serial port cable of the PL2303, because the PL2303 is cheap, so there are many counterfeit parallel imports, which will cause the download circuit to be unstable or even unable to download normally. At the same time, the disadvantage of this solution is that a 9-pin serial port interface must be made on the circuit board, which is a bit troublesome.

Method Two: 

        Please see Figure 2, this is the single chip computer download circuit. The effect is very good! Its advantage is that you only need to find an ordinary USB2.0 cable (as shown in Figure 2②) to download, which is much easier than finding a USB to serial cable. At the same time, because the serial port 9-pin interface is replaced by a USB interface, the volume and weight of the circuit board are also reduced a lot, and it also looks more beautiful and fashionable, hehe. Here I emphasize one point: it is recommended to use the CH340 chip (CH340G can be used, it has a large pin spacing and is easy to weld) instead of PL2303. Moreover, the peripheral circuit of PL2303 is relatively complex, which also increases the difficulty of debugging the hardware circuit. Of course, it is understandable to use PL2303. The circuit structure of Figure 2 is much simpler than that of Figure 1, so does it mean that the performance of Figure 2 is better than that of Figure 1? It should be! The probability of problems with simple structures is of course smaller!

        It is worth mentioning that when using CH340, there are some issues that need to be paid attention to, because many students fail to download programs using CH340. There are three common reasons:

        1. Some of the CH340 pins supply power to the MCU, causing the MCU to not be completely powered off (downloading the program requires powering off and restarting), which results in the inability to download the program. Solution: Connect a diode in series with the TXD pin of the CH340 (Schottky diode is recommended) and a resistor of about 300 ohms in series with the RXD pin of the CH340. In this way, the MCU can be completely powered off and the problem is solved! ! (Special note: This situation does not apply to chips such as stm32)

        2. When some students power the microcontroller, in order to prevent the reverse connection protection circuit, they connect a diode in series at the power supply. Due to the forward voltage drop of the diode, the microcontroller is underpowered, which makes the program unable to download. Solution: Replace the diode with a 0 ohm resistor or use a diode with a very small forward voltage drop to ensure the normal power supply of the microcontroller. Problem solved! !

              3. Also pay attention to the TXD pin and RXD pin. The correct connection method is to connect the TXD pin of CH340 to the RXD pin of the microcontroller, and the RXD pin to the TXD pin of the microcontroller.

        Finally, a typical application diagram of CH340 (taken from STC's document) is attached for reference!

     Let me share the USB serial port download circuit of STM32