C51 Programming Summary - Strange Knowledge of MCU 1 (IC Input and Output Voltage)

In the process of using the microcontroller, we must have a profound understanding of the "0" and "1" of the digital signal. Generally speaking, the high level "1" is 5V (for the microcontroller with 5V VCC), and the low level "0" is 0V.

In fact, we may use a chip with a power supply voltage of 3.3V, and the voltage of the high level "1" is 3.3V. When it exchanges data with the microcontroller, can the 5V microcontroller normally recognize the "1" of the 3.3V chip? 

Whether a 3.3V chip can exchange data normally with a 5V chip depends on the input and output voltage of the IC (also often called the I/O identification voltage), not VCC. Instead, it depends on the input and output voltage. We can check the relevant parameters in the IC specification sheet. The following are some electrical parameters of CH340.  

From the table, we can know that there are four parameters related to input and output voltage, which are

Note: To check the input and output voltages, you need to check the input and output voltages of the two chips, and the input and output of the two chips need to be checked crosswise.

The figure below shows the electrical characteristics of the 51 microcontroller

Since the VOL (<0.5V), VOH (>4.5V) of CH340, and the VIL (<0.8V), VIH (>2.0V) of C51 MCU, that is, the input and output levels of CH340 are both within the input and output level range of C51, the C51 MCU can normally identify the high and low levels of the output of CH340.

Assume that due to the VOL (<1V) of CH340, that is, the low level output of CH340 may be 1V, and the VIL (<0.8V) of the C51 microcontroller must be less than 0.8V to recognize the low level, so C51 may not recognize the low level.

Note: Generally speaking, two ICs with the same supply voltage can be recognized normally, and problems are usually caused by ICs with different supply voltages. However, there may be exceptions, so it is critical to determine the input and output voltages, which is the basis for whether the two ICs can communicate normally.