What Is the Minimum System of Single Chip Microcomputer

For 51 series single chip microcomputer, the minimum system should generally include: single chip microcomputer, crystal oscillator circuit and reset circuit. The following is a minimum system circuit diagram of 51 single chip microcomputer.

explain

Reset circuit: it is composed of capacitor series resistance. According to the figure and the property of "capacitor voltage cannot change suddenly", when the system is powered on, the rst pin will have a high level, and the duration of this high level is determined by the RC value of the circuit. The typical 51 single chip microcomputer will reset when the high level of RST pin lasts for more than two machine cycles. Therefore, the reliable reset can be guaranteed by properly combining the value of RC. General textbooks recommend that C take 10u and r take 8.2k. Of course, there are other methods. The principle is to make the RC combination produce a high level of no less than 2 machine cycles on the rst pin. As for how to calculate quantitatively, you can refer to books related to circuit analysis.

Crystal oscillator circuit: the typical crystal oscillator is 11.0592Mhz (because 9600 baud rate and 19200 baud rate can be obtained accurately, which is used for occasions with serial communication) / 12Mhz (to generate accurate US level time interval, which is convenient for timing operation)

Single chip microcomputer: one AT89S51 / 52 or other 51 series compatible single chip microcomputer

Special note: for pin 31 (EA / VPP), when the high level is connected, the MCU starts from 0000H of the internal ROM after reset; When the low level is connected, the reset is directly executed from the 0000H of the external ROM. This is easy for beginners to ignore.

Reset circuit

MCU reset circuit is like the restart part of the computer. When the computer crashes in use, press the restart button, and the program inside the computer will be executed from scratch. Single chip microcomputer is the same. When the single chip microcomputer system is running and the program flies due to environmental interference, press the reset button and the internal program will be executed automatically from the beginning.

The reset circuit of single chip microcomputer is shown as follows:

It is introduced in the book that 51 single chip microcomputer can be reset only by connecting a high level at pin 9 for 2us. How is this process realized?

In the single chip microcomputer system, the system resets once when it is powered on and started. When the key is pressed, the system resets again. If it is pressed again after release, the system will reset. Therefore, the reset can be controlled in the running system through the opening and closing of the key.

Why is it reset when starting up

In the circuit diagram, the capacitance is 10uF and the resistance is 10K. Therefore, according to the formula, it can be calculated that the time required for the capacitor to charge to 0.7 times the power supply voltage (the power supply of the single chip microcomputer is 5V, so charging to 0.7 times is 3.5V) is 10K * 10uF = 0.1s.

In other words, within 0.1s of computer startup, the voltage at both ends of the capacitor increases at 0 3.5V. At this time, the voltage at both ends of 10K resistor decreases from 5 1.5V (the sum of the voltages at all parts of the series circuit is the total voltage). Therefore, within 0.1s, the voltage received by RST pin is 5V 1.5V. In 51 single chip microcomputer with 5V working normally, the voltage signal less than 1.5V is a low-level signal, while the voltage signal greater than 1.5V is a high-level signal. Therefore, the MCU system will reset automatically within 0.1s after startup (the time of high-level signal received by RST pin is about 0.1s).

Why does it reset when the key is pressed

After the MCU is started for 0.1s, the voltage at both ends of capacitor C is continuously charged to 5V. At this time, the voltage at both ends of 10K resistor is close to 0V, and RST is at low level, so the system works normally. When the key is pressed, the switch is turned on. At this time, a circuit is formed at both ends of the capacitor, and the capacitor is short circuited. Therefore, in the process of pressing the key, the capacitor begins to release the charge before charging. Over time, the voltage of the capacitor changes from 5V to 1.5V or even less within 0.1s. According to the series circuit voltage is the sum of all parts, at this time, the voltage at both ends of 10K resistor is 3.5V or even greater, so RST pin receives high level again. The single chip microcomputer system automatically resets.

Summary

1. The principle of the reset circuit is that the rst pin of the single chip microcomputer receives a level signal above 2us. As long as the charge and discharge time of the capacitor is greater than 2us, the reset can be realized, so the capacitance value in the circuit can be changed.

2. Press the key to reset the system, which is caused by the capacitor in a short circuit circuit, releasing all electric energy and increasing the voltage at both ends of the resistance.

Introduction of minimum system circuit of 51 single chip microcomputer

1.51 the polarity capacitance C1 of the minimum system reset circuit of single chip microcomputer directly affects the reset time of single chip microcomputer. Generally, 10 30uf is used. The larger the minimum system capacitance of 51 single chip microcomputer is, the shorter the reset time is required.

2.51 the minimum system crystal oscillator Y1 of single chip microcomputer can also use 6mhz or 11.0592Mhz. Under normal working conditions, the crystal oscillator with higher frequency can be used. The oscillation frequency of the minimum system crystal oscillator of 51 single chip microcomputer directly affects the processing speed of single chip microcomputer. The greater the frequency, the faster the processing speed.

3.51 the minimum system starting capacitance C2 and C3 of single chip microcomputer are generally 15 33pf, and the closer the capacitance is to the crystal oscillator, the better the crystal oscillator is to the single chip microcomputer. Port P0 is an open drain output. When it is used as the output port, a pull resistance needs to be added, and the resistance value is generally 10K.

When the timer mode is set, adding 1 counter counts the internal machine cycles (one machine cycle is equal to 12 oscillation cycles, that is, the counting frequency is 1 / 12 of the crystal oscillation frequency). The count value n multiplied by the machine cycle tcy is the timing time t.

When set to counter mode, the external event count pulse is input to the counter by t0 or T1 pin. The t0 and T1 pin levels are sampled during s5p2 of each machine cycle. When a high level input is sampled in one cycle and a low level is sampled in the next cycle, the counter is incremented by 1, and the updated count value is loaded into the counter during s3p1 of the next machine cycle. Since it takes 2 machine cycles to detect a falling edge from 1 to 0, the sampled level is required to maintain at least one machine cycle. When the crystal oscillator frequency is 12Mhz, the maximum counting frequency shall not exceed 1 / 2MHz, that is, the period of counting pulse shall be greater than 2ms.