USB to serial communication

　　With the development of technology, RS232 serial communication is still widely used in industry. However, in the application of commercial technology, USB to UART technology has gradually replaced RS232 serial port. Most laptops no longer have serial port. So how can we realize communication between microcontroller and computer?

We only need to add a USB to serial port chip to our circuit to successfully realize the conversion between USB communication protocol and standard UART serial communication protocol. On our development board, we use the CH340T chip, as shown in Figure 1.

Figure 1 USB to serial port circuit

　　J2 on the left is a set of jumpers, which can be found in the lower left corner of our board. We short-circuit pins 3 and 5, and pins 4 and 6 through jumper caps. The circuit of CH340T on the right is very simple. After connecting the power circuit and crystal oscillator circuit, the DP and DM of pins 6 and 7 are connected to the two data pins of the USB port respectively, and pins 3 and 4 are connected to the TXD and RXD of our microcontroller through jumpers.

　　A 4148 diode is added to the 3rd pin position in the CH340T circuit, which is a little trick. Because our STC89C52RC microcontroller needs a cold start to download the program, that is, click download first and then power on. At the moment of power on, the microcontroller will first detect whether the program needs to be downloaded. Although the VCC of the microcontroller is controlled by the switch, since the 3rd pin of the CH340T is an output pin, if there is no diode, when the power is off in the back-stage microcontroller of the switch, the 3rd pin of the CH340T is connected to the P3.0 (ie RXD) pin of the microcontroller, and current will flow through this pin into the back-stage circuit and charge the capacitor of the back-stage, causing a certain voltage in the back stage. Although this voltage value is only about two or three volts, it may affect our cold start. After adding the diode, on the one hand, it does not affect communication, and on the other hand, it can eliminate this problem. This place can be used as a temporary understanding. If you make this circuit yourself, you can refer to it.