The 51 MCU contains a central processing unit (CPU) responsible for executing operations and managing control. It features four parallel I/O ports—P0, P1, P2, and P3—that allow communication with external devices. The microcontroller also includes ROM for program storage and RAM for temporary data. Additional components include timer/counter modules, serial I/O ports, an interrupt system, and an internal clock circuit.
Certain memory locations in the microcontroller are designated as Special Function Registers (SFRs), which are used to control these peripherals. There are 21 such SFRs in the 51 series, with some added in the 52 series marked with an asterisk (*). These registers can be directly addressed by the programmer and play a crucial role in controlling the microcontroller’s functions.
Here is a detailed breakdown of key SFRs:
1. **ACC (Accumulator)**
This register is essential for arithmetic and logical operations. It typically holds one operand during calculations. The accumulator has a zero flag (Z), which is set to 1 if the value is 0, and 0 otherwise. This flag is often used for conditional branching in programs.
2. **B Register**
Used primarily in multiplication and division operations. While not necessary for basic operations, it becomes important when handling more complex arithmetic tasks.
3. **PSW (Program Status Word)**
This register stores various flags that reflect the current state of the CPU. It includes:
- **CY (Carry Flag):** Indicates whether there was a carry or borrow in arithmetic operations.
- **AC (Auxiliary Carry Flag):** Reflects carry between the lower and upper nibbles.
- **F0:** A user-defined flag.
- **RS1, RS0:** Select the working register bank.
- **OV (Overflow Flag):** Indicates overflow in signed arithmetic.
- **P (Parity Flag):** Shows the parity of the number of 1s in the accumulator.
4. **DPTR (Data Pointer)**
Composed of DPH (high byte) and DPL (low byte), this 16-bit register is used to access external data memory. It can also serve as a general-purpose register when not needed for addressing.
5. **P0, P1, P2, P3 (I/O Ports)**
These four 8-bit ports provide input/output functionality. Their contents correspond to the voltage levels on their respective pins, allowing interaction with external hardware.
6. **IE (Interrupt Enable Register)**
Controls the enabling of interrupts. Bits like EA (Global Interrupt Enable), ET0, ET1, ES, EX0, and EX1 manage different interrupt sources.
7. **IP (Interrupt Priority Register)**
Determines the priority of each interrupt source. Higher-priority interrupts can preempt lower ones.
8. **TMOD (Timer Mode Register)**
Configures the operating mode of timers T0 and T1. It sets whether they function as timers or counters and defines their mode of operation.
9. **TCON (Timer Control Register)**
Manages the start and stop of timers, as well as overflow flags and interrupt requests. It also handles external interrupt triggers.
10. **SCON (Serial Communication Control Register)**
Controls the serial port's operation modes, including baud rate settings, data transmission, and reception. It also manages multi-machine communication and interrupt flags.
11. **PCON (Power Control Register)**
Used mainly in CMOS-based MCUs to control power-saving modes. It includes the SMOD bit, which doubles the serial baud rate when set.
12. **T2CON (Timer 2 Control Register)**
Configures Timer 2, allowing it to operate as a timer, counter, or baud rate generator. It includes flags for overflow, external interrupts, and mode selection.
These registers form the backbone of the 51 MCU, enabling efficient control over its operations, timing, and communication capabilities. Understanding them is essential for effective programming and system design.
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