Clock in

Digital Systems

Combinational logic circuit

A combinational logic circuit is one whose outputs depend only on its current inputs.

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Combinational Circuit

• A logic circuit whose outputs depend only on its current inputs is called a combinational circuit.

Combinational Circuit

• Logic circuit using AND, OR and NOT gates:

Sequential Logic Circuits

Sequential Logic Circuits• A Sequential Logic Circuit is one whose outputs

depend not only on its current inputs, but also on the past sequence of inputs.

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Sequential Logic Circuits

• The circuit controlled by the channel-up and channel-down pushbuttons on a TV is a sequential circuit.

• The channel selection depends on the past sequence of up/down pushes and the current push.

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Sequential Logic Circuits

• Outputs depend on the sequence of past inputs.

• As a result, the circuit must “remember” something about the past.

Sequential Logic Circuit

• A circuit with memory, whose outputs depend on the current input and the sequence of past inputs, is called a sequential circuit.

• The behaviour of such a circuit may be described by a state table that specifies its output and next state as functions of its current state and input.

Types of sequential circuits

• There are two main types of sequential circuits:– synchronous

– asynchronous

• their classification depends on the timing of their signals.• A synchronous sequential circuit is a system whose

behaviour can be defined from the knowledge of its signals at discrete instants of time.

• The behaviour of an asynchronous sequential circuit depends upon the order in which its input signals change and can be affected at any instant of time.

Asynchronous Sequential Logic Circuit• An asynchronous sequential circuit uses ordinary

gates and feedback loops. Propagation delay of these gates provide the needed memory, thereby creating sequential-circuit building blocks such as latches and flip-flops that are used in higher-level designs.

SR Flip-flop with Control Input

SR Flip_Flop with Clock input

• Q is an abbreviation of Q(t), referred to as the present state.

• Q(t+1), referred to as the next state after application of a single pulse at the clock input.

Synchronous Sequential Logic Circuit

• A synchronous sequential circuit , by definition, must employ signals that affect the memory elements only at discrete instants of time.

• One way of achieving this goal is to use pulses of limited duration throughout the system.

• the device that generates these pulses are called clock and

• the train of pulses are called clock pulses.

Clock

• The state changes of most sequential circuits occur at times specified by a free-running clock signal.

• Active high clock signal:

Clock

• Active low clock signal:

Clock period and frequency

• The clock period is the time between successive transitions in the same direction.

• The clock frequency is the reciprocal of the period. i.e., the number of clock pulses per second.

f = 1/T

Duty cycle

• Duty cycle is the percentage of time that the clock signal is at its asserted level.

Pentium4

• What is the clock frequency of the latest Pentium4 processor?

• What is the clock period of the latest Pentium4 processor?

Motherboard of a Pentium4 System

• What is the clock frequency of the Motherboard of a Pentium4 System?

• What is the clock period of the Motherboard of a Pentium4 System?

Standard Graphic Symbols for Latch and Flip-Flops

Flip-flops

• A device that stores either a 0 or 1.

• Stored value can be changed only at certain times determined by a clock input.

• New value depend on the current state and it’s control inputs.

• A digital circuit that contains filp-flops is called a synchronous sequential circuit.

Flip-flops

S-R latch symbols D flip-flop

J-K flip-flops

Microprocessors?

If the registers of a microprocessor are made of Flop-flops, what kind of digital circuits are they?

• Combinational Logic Circuits

• Sequential Logic Circuits

• Asynchronous Sequential Logic Circuits• Synchronous Sequential Logic Circuits

What Signal?

• Synchronous Sequential Logic Circuits require a _______________ signal to operate.

• In general, all digital Circuits require a _______________ signal to operate.

Pins of 8284 - Clock Generator and Driver

Inputs to 8284

• X1 and X2 (crystal in)

– X 1 and X2 are the pins to which a crystal is attached. – The crystal frequency must be 3 times the desired

frequency for the microprocessor. – The maximum crystal for the 8284A is 24 MHz and

30 MHz for the 8284A-1. – The IBM PC is connected to a crystal of 14.31818

MHz. – For some turbo compatibles, it is 24 MHz.

Outputs of 8284

• CLK (clock)

– This is an output clock frequency equal to one-third of the crystal oscillator, or EFI input frequency, with a duty cycle of 33%.

– This is connected to the clock input of the 8088/86 and all other devices that must be synchronized with the CPU .

– In the IBM PC it is connected to pin 19 of the 8088 microprocessor and other circuitry under the CLK88 label.

– This frequency, 4.772776 MHz ( 14.31818 divided by 3), is the processor frequency on which all of the timing calculations of the memory and I/O cycle are based.

Which system transfers fastest?

Every clock the data buses transfer 1 Byte of data on both systems.

• System-A is running at a clock frequency of 2 GHz.

• System-B is running at a clock frequency of 3 GHz.

Which system transfers fastest?

Every clock the data buses transfer 1 Byte of data from the memory to the processor on both systems.

• System-A is running at a clock frequency of 100 MHz.

• System-B is running at a clock frequency of 133 MHz.

What are their Memory access time?

Every clock the data buses transfer 1 Byte of data from the memory to the processor on both systems.

• System-A is running at a clock frequency of 100 MHz.

• System-B is running at a clock frequency of 133 MHz.