The control unit (CU) generates timing and control signals that coordinate operations in the computer. It controls data flow between the processor, memory, and peripherals, and directs the system to carry out instructions. Timing is controlled by a master clock generator that applies signals to registers and flip-flops. The CU generates control signals that provide inputs to the multiplexer on the common bus and processor registers. There are two major types of control organization: hardwired control uses gates and digital circuits while microprogram control stores control information in a control memory that is initialized to produce a sequence of micro-operations and can be modified by updating the microprogram. A memory read/write cycle is initiated on the rising edge of a

1. Timing and Control Circuit
 Name- Rajan Shah

2. Control Unit
 The CPU is partitioned into two parts viz:
1. CU-Control Unit
2. ALU- Arithmetic Logic Unit
 The function of the CU is to generate relevant
timing and control signals to all the operations
in the computer.
 It controls the flow of data between processor,
memory and peripherals.
 CU directs the entire computer system to carry
out stored program instructions.

3. Timing and Control Signals
 The timing for all the registers in the basic
computer is controlled by the Master Clock
Generator and applied to all the flip-flops and
registers in the system.
 The control signals are generated in control unit
& provide control inputs for the Mux in common
bus, control inputs in the processor register and
micro-operations for accumulator.

4. Type of Major Control Organization
I. The control logic is
implemented with the
gates, FF’s, decoders &
other digital circuits.
II. It can be optimised to
produce fast mode of
operation.
III. Changes in the wiring
among various
components is required
for modification.
I. The control information
is stored in the control
memory.
II. The control memory is
initialized to produce
sequence of micro-
operations.
III. Modification can be
done by updating the
micro program in the
control memory.
Hardwire Control
Micro program
control

5. Timing and Control Unit

6.  A memory read-write cycle will be initiated with
the rising edge of timing signal.
 It will be assumed that a memory cycle time is
less than clock cycle time, according to which
memory cycle will be completed & the clock
transition will be used to load the memory word
into a register.
 Usually wait cycles are provided in the
processor until memory word is available as
memory cycles usually take longer than
processor clock cycle.

7. Timing Graph

8.  To understand the operation of basic
computer, it is necessary to understand the
timing relationship between clock transition
and timing signals.
 For E.g.- T0 : AR  PC.
 The above register transfer statement
specifies a transfer of content of PC to AR if
timing signal T0 is active.
 During T0, all the contents of PC are placed
onto bus(S2S1S0 = 010) & LD(load) input of AR
is enabled.

9.  The transfer doesn’t occur until the end of
clock cycle when a clock goes through a
positive transition and also increments the SC
from 0000 to 0001 which has T0 inactive and
T1 active.

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