1. Reset condition
The 89C51 microcontroller reset requires a high level of up to 24 clock cycles to reset. The reset function is to make the program pointer point to address 0. Each program starts from address 0, so the concept of reset is to let the program start from scratch. carried out.
2. Reset circuit schematic
3. Principle description
The reset circuit has a power-on reset function, which is implemented by C1 (polarity capacitance). When the system is powered on, C1 has a charging and discharging process, and a high level is generated during the discharging process. The discharging time is calculated according to the formula (t=RC squared). R is the resistance of the resistor R2, and C is the magnitude of the polarity capacitor C1.
When the system is running normally, when the button S1 is pressed, the level of the RST terminal is VCC*10/11, which is also a high level. At this time, the chip also generates a high level reset signal.
4. About the concept of clock
The concept of machine cycle and instruction cycle:
Oscillation period: Also called clock period, it refers to the period of the oscillation source that provides the clock signal to the microcontroller.
Status Period: Each status period is twice the clock period, which is obtained by dividing the oscillation period by two.
Machine cycle: One machine cycle consists of six status cycles S1~S6, which is 12 clock cycles. The CPU can perform a separate operation in one machine cycle.
Instruction cycle: It refers to the total time required for the CPU to complete an operation. Each instruction execution time consists of one or several machine cycles. In the MCS-51 system, there are single-cycle instructions, two-cycle instructions, and four-cycle instructions.
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