The 8051 microcontroller is a popular 8-bit microcontroller widely used in embedded systems. Its timing and control unit (TCU) is responsible for generating and controlling the timing signals required for the proper functioning of the microcontroller.
Functions
Here are the main functions of the 8051 timing and control unit:
- Clock Generation: The TCU generates the internal clock signal for the microcontroller. The clock signal synchronizes the operation of various components within the microcontroller, ensuring proper timing and execution of instructions.
- Oscillator Control: The TCU provides control over the oscillator circuitry of the 8051 microcontroller. It allows the selection of different clock sources, such as an external crystal or an internal oscillator, depending on the specific configuration.
- Instruction Cycle Timing: The TCU determines the timing of the instruction cycle in the microcontroller. It controls the duration of each machine cycle and provides the necessary timing signals, such as the oscillator frequency and the number of clock cycles required to execute instructions.
- Timer/Counter Control: The TCU is responsible for controlling the operation of the built-in timer/counter modules in the 8051 microcontroller. It configures the timers and counters for different modes of operation, sets the initial values, and generates interrupts when specific conditions are met.
- Interrupt Control: The TCU manages the interrupt system of the 8051 microcontroller. It enables or disables interrupts, prioritizes interrupt requests, and handles the necessary interrupt-related operations, such as saving and restoring the context of the interrupted program.
- Power Management: In some variants of the 8051 microcontroller, the TCU also provides power management features. It controls the power modes of the microcontroller, allowing it to enter low-power states to conserve energy when not actively processing instructions.
Pin Functions
Pin 9: RST
- It is an active high input pin and normally low. Upon applying a high pulse to this pin, 8051 terminate all activities and go to reset condition. By applying logic zero to this pin, the program starts execution from the beginning.
- 8051 responds to an RST high input only if the RST is held high for at least two machine cycles.
- A machine cycle is the period taken by any processor to fetch and execute one instruction.
Pin 18, 19: X2, X1
- The 8051 has an on-chip oscillator but requires an external crystal to run it.
- A quartz crystal oscillator is connected to inputs XTAL1 (pin19) and XTAL2 (pin18).
- The quartz crystal oscillator also needs two capacitors of 30 pF value. The original 8051 operates at 12 MHz.
- The crystal frequency determines the basic clock frequency of 8051. A quartz crystal which specifies operating frequency is usually connected to these pins.
Pin 29: 
If external ROM is used for storing program then a logic zero (0) appears on it every time the microcontroller reads a byte from memory.
Pin 30: ALE
- Prior to reading from external memory, the microcontroller puts the lower address byte (A0-A7) on P0 and activates the ALE output.
- After receiving signal from the ALE pin, the external register (usually 74HCT373 or 74HCT375 add-on chip) memorizes the state of P0 and uses it as a memory chip address. Immediately after that, the ALU pin is returned its previous logic state and P0 is now used as a Data Bus.
- As seen, port data multiplexing is performed by means of only one additional (and cheap) integrated circuit. In other words, this port is used for both data and address transmission.
Pin 31:
- By applying logic zero to this pin, P2 and P3 are used for data and address transmission with no regard to whether there is internal memory or not.
- It means that even there is a program written to the microcontroller, it will not be executed. Instead, the program written to external ROM will be executed.
- By applying logic one to the EA pin, the microcontroller will use both memories, first internal then external (if exists).
In 8051, the maximum number of clock cycles taken for a machine cycle is 12. So, the RST pin must be high for at least 24 clock periods.
Overall, the timing and control unit of the 8051 microcontroller plays a crucial role in generating timing signals, controlling the operation of timers/counters, managing interrupts, and ensuring proper timing and synchronization of the microcontroller’s components.