Sequential Logic. (Materials taken from: Principles of Computer Hardware by Alan Clements )

Transcription

1 Sequential Logic (Materials taken from: Principles of Computer Hardware by Alan Clements )

2 Sequential vs. Combinational Circuits Combinatorial circuits: their outputs are computed entirely from their present inputs Sequential circuits: their outputs are computed using both the present input and their previous output The idea of a next state A clock input triggers the transition from the current state to next state.

3 The Big Picture

4 Sequential Logic Elements Sequential logic can perform as many different functions as combinational logic Certain functions have been given names. Latch: a 1-bit memory element Register : m latches in a row and is able to store an m-bit word Shift register: special-purpose register that moves the bits of the word left or right Counter: special-purpose register that when triggered (i.e. clocked) increases its contents by 1 State machines: moves from one state to another each time it is triggered

5 The Flip-Flop Basic building block of sequential circuits

6 Analysis by Assuming Initial Conditions

7 Truth Table

8 RS Flip-Flop

9 NAND Gate Implementation

10 Clocked RS Flip-Flop NOR gate implementation NAND gate implementation

11 D Flip-Flop D = 1 sets the RS flip-flop and D = 0 clears it

12 D Flip-Flop Chip 2 active low inputs called preset and clear - Unconditional - Set high in normal usage - Used on power-up

13 Registers Registers transmit and receive data from buses Flip-flops are clocked to latch the data

14 Registers The most common sequential building block is the register. A register is N bits wide and has a load line for loading in a new value into the register. Note that DFF simply loads old value when LD = 0. DFF is loaded devery clock cycle. Copyright Thomson/Delmar Learning, All rights reserved. V

15 Ways of Clocking a Flip-Flop A clocked flip-flop captures a digital value and holds it; three ways of clocking: 1. Whenever the clock is asserted (i.e. a leveltriggered flip-flop). 2. Whenever the clock is changing state (i.e. an edge-triggered flip-flop). 3. Capture data on one edge of the clock and transfer it to output on the following edge (i.e. a master slave flip-flop)

16 Level-Triggered FF The type of FF we have discussed so far Not appropriate for all applications

17 Edge-Triggered FF An edge-triggered flip-flop is clocked not by the level of the clock (i.e. high or low), but by the transition of the clock signal from zero to one, or one to zero. Positive or rising-edge sensitive Negative or falling-edge sensitive The rising or falling edge of a pulse can have durations less than 1 ns Like an level-sensitive clock triggered by a infinitesimally short pulse

18 Edge-Triggered FF Positive edge-triggered D flip-flop

19 Edge-Triggered FF Variation in the arrival time of clock pulses to FF clock inputs is called clock skew Caused by differences in the paths by which clock pulses reach flip-flops Distance Temperature Age

20 Master-Slave FF

21 Timing Diagrams

22 JK Flip-Flop Edge-triggered flip-flops When a JK flip-flop is clocked, it behaves like an RS flip-flop (where J = S, K = R) for all input conditions except J = K = 1

23 Summary Flip-flops have internal state as well as external inputs---they are memory elements The most common forms of flip-flop are: D flip-flop, RS flip-flop, and JK flip-flop Flip-flops have two outputs: Q and Q Most flip-flops are clocked and the clock is used to trigger the flip-flop. Flip-flops often have unconditional preset and clear inputs used the set or clear the output

24 Shift Register Shift register: bits can be moved one place right every time the register is clocked Example, the binary pattern becomes after the shift register is clocked once and after it is clocked twice and after it is clocked three times, and so on After the first shift a 0 has been shifted in from the lefthand end and the 1 at the right-hand end has been lost Shifting 1 place right has the effect of dividing the number by 2; 1 place left corresponds to multiplying by 2

25 Right-Shift Register Using D FFs

26 Shift Register Very useful sequential building block. Used to perform either parallel to serial data conversion or serial to parallel data conversion. Copyright Thomson/Delmar Learning, All rights reserved. V

27 Binary Counter Using JK Flip-Flops

28 Counter Very useful sequential building block. Used to generate memory addresses, or keep track of the number of times a datapath operation is performed. Copyright Thomson/Delmar Learning, All rights reserved. V