Control Circuit Design Lecture Notes

Overview

• Control circuit can be designed in two ways:
  1. Hardwired Control Unit
  2. Microprogrammed Control Unit

Purpose of the Control Unit

• The primary goal is to generate signals that direct operations within a circuit.

Hardwired Control Unit

Characteristics

• Circuit Design: Focus on designing the circuit for the control unit.
• Input Sources: Control unit decisions depend on inputs from:
  • Instruction Register: Holds opcode bits (e.g., add, load instructions).
  • Step Counter: Helps in determining the current processing stage.

Inputs for Control Unit

1. Instruction Register

   • Stores opcode bits to determine the type of instruction.
   • Instruction Decoder processes opcode:
     • Generates signals (e.g., i1, i2, i3) for control signal generation.

2. Step Counter

   • Indicates the current stage of instruction execution:
     1. Fetch
     2. Decode
     3. Execute (ALU)
     4. Memory Access
     5. Write Back
   • Each stage typically takes one clock cycle.
   • The counter cycles through values (0-4) to indicate the current stage.

Clock and Counter Mechanism

• Clock Generator: Generates periodic clock signals to drive the step counter.
• Counter Functionality:
  • Increment on each clock cycle to reflect the current stage of the operation.
  • Example sequence: 0 (Fetch) -> 1 (Decode) -> 2 (ALU) -> 3 (Memory) -> 4 (Store)
    • After reaching 4, it returns to 0 for the next instruction cycle.

Conditional Signals

• Generated during ALU operations (e.g., Negative bit, Zero bit, Overflow bit) that can alter instruction flow.
• These signals need to be incorporated in the control signal generation.

External Inputs

• Inputs from external sources (e.g., keyboard interrupts) can affect the control signal generation and flow of operations.

Step Counter Enable Mechanism

• Counter Enable Signal: Controls whether the step counter increments.
• Memory Access: If memory access takes longer than one clock cycle:
  • Counter enable signal can be set to 0 to stall the process until memory operation completes.
  • Memory Function Complete (MFC) signal from memory interface is used to reset the counter enable to 1.

Conclusion

• Understanding the functionality of a hardwired control unit:
  • Instruction input from instruction register
  • Stage tracking via step counter
  • Conditional and external signals management
• Importance of designing robust control circuits that can handle various operational delays and signal generation needs.