What is the minimum single-chip microcomputer system?

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} "> Minimum MCU {
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} "> System, or minimum {
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} "> An application system is a system that can work with a single-chip microcomputer consisting of the least components.
For 51 series single-chip microcomputer, the minimum system should generally include: single-chip microcomputer, crystal oscillator {
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} "> Circuit, reset circuit.
The following is a minimum system of 51 Single-Chip Microcomputer {
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} "> Circuit diagram.

Description
Reset Circuit: {
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} "> Capacitor series {
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} "> The resistor is composed of a graph and a" capacitor {
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} "> The nature of voltage cannot change suddenly", you can know that when the system is powered on, the RST foot will appear high, and this high level continues {
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} "> The time is determined by the RC value of the circuit. A typical 51 single-chip microcomputer is reset when the high level of the RST foot continues more than two machine cycles. Therefore, a proper combination of RC values can ensure reliable reset. general Textbook {
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} "> We recommend that C take 10u and r take 8.2k. of course there are other methods. The principle is that the RC combination can generate at least two high levels of machine cycles on the rst. for details about how to perform quantitative computing, refer to the relevant books on circuit analysis.
Crystal oscillator circuit: Typical crystal oscillator uses 11.0592 MHz (because 9600 baud rate and 19200 baud rate can be accurately obtained, it is used {
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} "> Serial port {
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} "> Communication scenarios)/12 MHz (precise US-level time breaks are generated to facilitate scheduled operations)
Single-Chip Microcomputer: single-chip microcomputer, which is compatible with the single-chip microcomputer of AT89S51/52 or other 51 series.
Special note: for 31-pin (Ea/VPP), when the power supply is high, the MCU starts to execute the swap H from the internal ROM after the reset; when the power supply is low, after resetting, it is executed directly from the memory H of the external Rom. this is easy for beginners to ignore.

Reset Circuit:
I. Purpose of the Reset Circuit
Single-Chip reset circuit is like the restart part of the computer. When the computer crashes during use, press the restart button {
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} "> The program starts from scratch. The same is true for single-chip microcomputer. When the single-chip microcomputer system is running and the program is running due to environmental interference, the internal program of the reset button is automatically executed from the beginning.
Microcontroller reset circuit such:

Ii. Reset Circuit operation {
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} "> Principle
I have introduced in books that 51 single-chip microcomputer can be reset only by connecting a High Level 2 us at the 9th pin. How can this process be achieved?
In the single-chip microcomputer system, the system is reset once when powered on and started, and the system is reset again when the key is pressed. If it is released and then pressed, the system will reset. Therefore, you can control the reset of a running system by disconnecting and closing the buttons.
Why is it reset at startup?
In the circuit diagram, the capacitance is 10 UF and the resistance is 10 KB. Therefore, according to the formula, you can calculate that the capacitor is charged {
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} "> The power supply voltage is 0.7 times (the single-chip microcomputer power supply is 5 V, so charging to 0.7 times is 3.5 V), the time required is 10 K * 10 UF = 0.1 s.
That is to say, within S of the Computer startup, the voltage at both ends of the capacitor is 0 ~ 3.5v increase. At this time, the voltage at both ends of the 10 k resistance is from 5 ~ V Reduction (sum of voltage in all areas of the series circuit and total voltage ). Therefore, within S, the voltage received by the RST pin is 5 V ~ 1.5 V. 51 Single-Chip Microcomputer in 5 V working normally is smaller than 1.5v voltage {
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} "> The signal is a low-level signal, while the voltage signal greater than 1.5v is a high-level signal. Therefore, the single-chip microcomputer system will automatically reset within S of the boot (the High-Level Signal received by the RST pin is about S ).
Why reset when pressing the key?
After the single-chip microcomputer starts for S, the voltage at both ends of the capacitor C is continuously charged as 5 V. This is when the voltage at both ends of the 10 k resistor is close to 0 V, and the RST is in a low level, so the system works normally. When the key is pressed ,{
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} "> 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 charge before the capacitor starts to be released. Over time, the capacitor voltage ranges from 5 V to 1.5 V, or even smaller, within S. According to the sum of the voltage of the series circuit, the voltage at both ends of the 10 k resistance is 3.5 V, or even greater, so the RST pin receives a high level. The single chip microcomputer system is automatically reset.

Summary:
1. The principle of the reset circuit is that the RST pin of the single-chip microcomputer receives a level signal greater than 2 us, as long as the charging and discharging time of the capacitor is greater than 2 us, the reset can be realized, therefore, the capacitance value in the circuit can be changed.
2. Press the button to reset the system. The capacitor is in a short-circuit, releasing all the power, and the voltage at both ends of the resistance increases.

Introduction to the minimum system circuit of 51 Single-Chip Microcomputer
1.51 The polarity capacitor C1 of the minimum single-chip microcomputer reset circuit directly affects the reset time of the single-chip microcomputer. Generally, 10 ~ 30 UF, 51 single-chip microcomputer, the larger the minimum system capacity, the shorter the reset time.
2.51 single-chip microcomputer minimum system Y1 can also use 6 MHz or 11.0592 MHz, while normal operation can use a higher frequency crystal oscillator, the oscillator frequency of the smallest single-chip microcomputer system directly affects the processing speed of the single-chip microcomputer. The higher the frequency, the faster the processing speed.
3.51 Single Chip Microcomputer minimum system vibration capacitor C2, C3 generally use 15 ~ 33pf, and the closer the capacitor is to the crystal oscillator, the closer the crystal oscillator is to the microcontroller. 4. Open or leak the P0 port {
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} "> The output port must contain a pulling resistance of 10 KB.
{
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} "> Set {
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} "> In the timer mode, adding a counter is used to count the cycle of the internal machine (one cycle equals 12 oscillation cycles, that is, the counting frequency is 1/12 of the crystal oscillator frequency ). The Count value n multiplied by the machine cycle TCY is the scheduled time t.

When set to counter mode, the external event Count pulse consists of T0 or t1 pin {
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} "> Enter the counter. Sample T0 and T1 pin levels during s5p2 of each machine cycle. When a period is sampled to a high-level input, and the next period is sampled to a low electric frequency, the counter is incremented by 1, and the updated counting value is loaded into the counter during the s3p1 of the next machine cycle. Because {
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} "> Detecting a descent from 1 to 0 requires two machine cycles. Therefore, the sampled level must be maintained at least one machine cycle. When the crystal oscillator frequency is 12 MHz, the maximum counter frequency cannot exceed 1/2 MHz, that is, the period of the counter pulse must be greater than 2 ms.