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Transcript of Clock in Digital Systems. Combinational logic circuit A combinational logic circuit is one whose...
![Page 1: Clock in Digital Systems. Combinational logic circuit A combinational logic circuit is one whose outputs depend only on its current inputs. 1432.](https://reader036.fdocuments.net/reader036/viewer/2022062315/5697c00c1a28abf838cc8c1a/html5/thumbnails/1.jpg)
Clock in
Digital Systems
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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.
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Combinational Circuit
• Logic circuit using AND, OR and NOT gates:
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Sequential Logic Circuits
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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.
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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.
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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.
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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.
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SR Flip-flop with Control Input
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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.
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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.
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Clock
• The state changes of most sequential circuits occur at times specified by a free-running clock signal.
• Active high clock signal:
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Clock
• Active low clock signal:
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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
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Duty cycle
• Duty cycle is the percentage of time that the clock signal is at its asserted level.
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Pentium4
• What is the clock frequency of the latest Pentium4 processor?
• What is the clock period of the latest Pentium4 processor?
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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?
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Standard Graphic Symbols for Latch and Flip-Flops
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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.
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Flip-flops
S-R latch symbols D flip-flop
J-K flip-flops
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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
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What Signal?
• Synchronous Sequential Logic Circuits require a _______________ signal to operate.
• In general, all digital Circuits require a _______________ signal to operate.
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Pins of 8284 - Clock Generator and Driver
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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.
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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.
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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.
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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.
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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.