"Turn" what is called 51 single-chip microcomputer minimum system

Single-chip microcomputer minimum system, or the smallest application system, refers to the system which is composed of the least component and the microcontroller can work. for the 51 series microcontroller, the minimum system should generally include: single-chip microcomputer, crystal oscillator circuit, reset circuit. a minimum system circuit diagram for a 51 microcontroller is given below.

DescriptionReset circuit: ByCapacitance seriesThe resistor consists of a combination of "capacitanceThe nature of the voltage cannot be mutated, it can be known that when the system is power on, the RST foot will appear high, and this high-level continuousThe time is determined by the RC value of the circuit. Typical 51 single chip microcomputer when the high level of the RST foot lasts two machine cycles above will be reset, so, the appropriate combination of RC value can guarantee a reliable reset. General textbook recommended C take 10u,r take 8.2K. Of course there are other, The principle is to allow the RC combination to generate a high level of not less than 2 machine cycles on the RST foot. As for how to calculate quantitatively, we can refer to the circuit analysis related books. Crystal circuit: Typical crystal oscillator 11.0592MHz (because it is accurate to get 9600 baud rate and 19200 baud rate for serial communication)/ 12MHz (produces accurate US-class break, convenient timing operation) special note: For 31 feet (EA/VPP), when the high-power, the microcontroller after the reset from the internal ROM 0000H start execution; The reset is performed directly from the 0000H of the external ROM. This is easy for beginners to ignore.
    reset circuit: One, the purpose of the reset circuit The microcontroller reset circuit is like the computer restart part, when the computer in use in the panic, press the restart button inside the computer

Second, the work of the reset circuitPrincipleIn the book is introduced, 51 single-piece confidential reset only need to be in the 9th pin to a high level of 2US can be achieved, then how is this process achieved?In the single-chip microcomputer system, the system resets once when the power is on, and when the button is pressed, the system resets again, if it is released and then pressed, the system will reset. It is possible to control the reset of the operating system by disconnecting and closing the keys.Why is the reset when bootingIn the circuit diagram, the size of the capacitor is 10uF, the size of the resistor is 10k. So according to the formula, we can calculate the capacitance to charge toThe supply voltage is 0.7 times times (the SCM power is 5V, so charging to 0.7 times times that is 3.5V), the time required is 10k*10uf=0.1s.In other words, the voltage at both ends of the capacitor increases in the 0.1S of the computer's starting 0~3.5v. At this time the voltage at both ends of the 10K resistor is reduced from 5~1.5v (the sum of the voltages across the series circuit is the total voltage). So within 0.1S, the voltage received by the RST pin is 5v~1.5v. Voltage less than 1.5V in a 51 single-chip microcomputer that works in 5VThe signal is a low-level signal, and a voltage signal greater than 1.5V is a high-level signal. So in the boot 0.1S, the microcontroller system automatic reset (RST pin received high-level signal time of about 0.1S).Why is it reset when the key is pressed?After the microcontroller starts 0.1S, the voltage on both ends of the capacitor C is continuously charged at 5 V, which is when the voltage at both ends of the 10K resistor is close to 0v,rst low so the system works properly. When the button is pressed,Switch conduction, this time the capacitor is formed on both ends of a circuit, the capacitance is shorted, so in the process of pressing the button, the capacitor starts to release the charge before the charge. Over time, the voltage of the capacitor is within 0.1S, from 5V to 1.5V, or even smaller. Depending on the voltage of the series circuit, the voltage at the 10K resistor at this time is 3.5V or greater, so the RST pin receives a high level. Single-chip microcomputer system automatic reset.
Summarize:1, the principle of the reset circuit is the MCU RST pin received more than 2US level signal, as long as the capacitor charge and discharge time is greater than 2US, you can achieve the reset, so the capacitance value in the circuit can be changed.2, the key press the system reset, is the capacitor in a short circuit, the release of all the Power, the voltage on both ends of the increase caused by.
Introduction of minimum system circuit of 51 single chip microcomputer1.51 single-chip microcomputer minimum system reset circuit of the polar capacitance C1 the size of the direct impact on the single-chip reset time, the general use of 10~30uf,51 single-chip microcomputer minimum system capacity of the greater the need for a shorter reset time.2.51 SCM minimum System crystal oscillator Y1 can also be used 6MHz or 11.0592MHz, in the case of normal operation can use higher frequency crystal oscillator, 51 single-chip microcomputer minimum system crystal oscillator oscillation frequency directly affect the processing speed of single-chip microcomputer, the more frequency The faster the large processing speed. 3.51 single-chip microcomputer minimum system starting capacitor C2, C3 general use 15~33PF, and the closer the capacitor from the crystal, the better the crystal oscillator from the microcontroller closer the better 4. P0 port for open-drain output, as an outlet need to add a pull resistor, the resistance is generally 10k. set to timer mode, the plus 1 counter is the internal machine cycle count (one machine cycle equals 12 oscillation cycles, i.e. the counting frequency is 1/12 of the crystal oscillator frequency). The Count value n times the machine cycle tcy is the timed time t.
input to the counter. T0, T1 pin levels are sampled during the s5p2 of each machine cycle. When a period is sampled to a high input, and the next cycle is sampled to a low level, the counter is incremented by 1, and the updated count value is mounted on the counter during the s3p1 of the next machine cycle. Because

"Turn" what is called 51 microcontroller minimum system