MCU Basics (III): Clock System and Reset

MCU clock system

  One is the internal clock oscillation mode, which requires a quartz crystal and an oscillation capacitor to be connected between the XTAL1 and XTAL2 pins; the other is the external clock mode, which requires XTAL1 to be grounded and the external clock signal to be introduced from the XTAL2 pin.

Machine cycle and instruction cycle

  Machine cycle: The basic operation cycle of a single-chip microcomputer. Each machine cycle contains 6 states: S1, S2, S3, ... S6. Each state contains two beats, P1 and P2. Each beat is a clock cycle (oscillation cycle). That is, a machine cycle contains 12 clock cycles, which can be expressed as S1P1, S1P2, S2P1, ... S6P1, S6P2 in sequence.

  Oscillation period = 1 second / crystal frequency, so the machine cycle of the microcontroller = 12 seconds / crystal frequency

  Instruction cycle: When a computer is working, it constantly fetches and executes instructions. The time required to fetch an instruction and execute it is called an instruction cycle. Different instruction cycles have different durations. Most instruction cycles consist of one or two machine cycles. Only multiplication and division instructions require four machine cycles.

Reset

  Today's single-chip microcomputers cannot guarantee that they will not crash. When the program is executed abnormally or the machine needs to be reset, the reset operation clock circuit is required. When the external circuit causes the RST terminal to have a high level for more than two cycles, the system is reset internally.

After reset, the microcontroller