The 89C51 microcontroller (MCU) operates in four main modes: reset, program execution, low power consumption, and programming/verification. Each mode serves a specific purpose in the overall functionality of the device. **1. Reset Mode** Reset is the initialization process of the MCU. Its primary function is to set the Program Counter (PC) to 0000H, allowing the MCU to start executing the program from the beginning of memory. In addition to initializing the system, a reset is often necessary when the system becomes unresponsive due to software errors or incorrect operations. Pressing the reset button helps restart the system and recover from such issues. During a reset, not only is the PC initialized, but several special function registers (SFRs) are also set to their default states, as shown in the table above. Additionally, certain pin signals on the microcontroller are affected. For example, the ALE and PSEN signals become inactive during reset, meaning ALE = 1 and PSEN = 1. **1.1 Reset Signal and Its Generation** The RST pin is used to receive the reset signal, which is active high. The reset signal must remain active for at least 24 oscillator cycles (or two machine cycles). If a 6MHz crystal is used, the reset duration should be more than 4μs to ensure proper initialization. The reset circuit consists of both internal and external components. The external circuit generates the RST signal, which is then processed by the Schmitt trigger. The internal reset circuit samples the output of the Schmitt trigger at S5P2 of each machine cycle to generate the internal reset signal. **1.2 Types of Reset Modes** There are three types of reset modes: - **Power-on Automatic Reset:** This is achieved through an external capacitor that charges when power is applied. As long as the VCC rise time is less than 1ms, the system will automatically reset upon startup. - **Button Manual Reset:** This involves using a switch connected between the RST pin and VCC through a resistor. When the button is pressed, it pulls the RST pin high, initiating a reset. The resistor and capacitor values are chosen to ensure the reset pulse lasts longer than two machine cycles. - **External Pulse Reset:** In this mode, a reset pulse is externally applied to the RST pin. The pulse must be wide enough to last for more than two machine cycles. After the pulse ends, an internal pull-down resistor ensures the RST pin returns to a low level. **2. Program Execution Mode** The program execution mode is the most common working state of the 89C51 MCU. After a reset, the PC is set to 0000H, so the MCU starts executing instructions from that address. However, most programs do not begin directly at 0000H. Instead, an unconditional jump instruction is typically placed at 0000H to redirect execution to the actual starting point of the program. This allows for better organization and flexibility in program design.

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