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Sequential circuits. part 2: implementation, analysis & design All illustrations 2009-2010, Jones & Bartlett Publishers LLC, ( More summer fashion. SR is one of 4 basic flip flops common in computer design Others can all be constructed from SR; they are:

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sequential circuits

Sequential circuits

part 2: implementation, analysis & design

All illustrations 2009-2010, Jones & Bartlett Publishers LLC, (

more summer fashion
More summer fashion
  • SR is one of 4 basic flip flops common in computer design
  • Others can all be constructed from SR; they are:
    • JK (don’t know why it’s called that)
    • D (data)
    • T (toggle)
jk flip flop
JK flip flop
  • Resolves undefined transition in SR
    • J input acts like S (sets device)
    • K acts like R (resets)
  • When JK = 11, have toggle condition: switch from one state to other
jk flip flop implementation
JK flip flop implementation
  • If JK = 00, SR = 00 because of AND – so SR won’t change state when clocked
jk flip flop implementation6
JK flip flop implementation
  • If JK = 10, R must be 0:
    • if Q=0, Q’=1, so SR=10, the set condition: flip flop will change state (to Q=1)
    • if Q=1, Q’=0, SR=00 (stable condition) so flip flop stays in Q=1
jk flip flop implementation7
JK flip flop implementation
  • If JK = 01, final state is Q=0 (analogous to JK=10)
jk flip flop implementation8
JK flip flop implementation
  • If JK=11, Q connects directly to R, Q’ to S
    • so if Q=0, SR=10, so Q=1
    • if Q=1, SR=01, so Q=0
d flip flop
D flip flop
  • D: data; one input + CP
    • Q(t+1) independent of Q(t) – depends only on value of D at time t
    • D flip flop holds data until next pulse
constructing registers
Constructing registers
  • Can use D flip flops to construct individual bits of registers – one signal sent to each bit
  • Setting/resetting flip flop requires a 1 signal on exactly one of its input lines – CP restricts incoming signal to appropriate time so device remains in sync
  • D is split in 2, with one half inverted – so always 1 true, 1 false on data line
  • Since CP usually false, both inputs normally 0 (no change in flip flop)
  • When clock goes high, one of 2 lines (S or R) delivers 1
device select signal
Device select signal
  • Used in combination with CP & D signals to determine if register should send or receive data
  • When one register is to send to another, 3 simultaneous signals sent to each register:
    • clock
    • device select
    • send or receive
  • All 3 ANDed together to indicate that specific register should send or receive at specific time
t flip flop
T flip flop
  • T stands for Toggle
    • like D, has one input + CP
    • acts like control line that specifies selective toggle
    • if T=0, flip flop doesn’t change; if T=1, toggles
implementation of t flip flop
Implementation of T flip flop
  • Identical to JK, with J=K
general sequential network
General sequential network
  • Sequential circuit: interconnection of gates & flip flops
  • All gates can be grouped conceptually as combinational network, all flip flops as group of state registers
  • Between clock pulses, combinational part produces output; amount of time needed depends on number of gates in net
general sequential network15
General sequential network
  • Arrows: one or more connecting lines
  • I/O lines: connections to external environment
  • Arrow between boxes: input lines to flip flops
  • Clock line assumed but not shown
hardware analysis vs design
Hardware analysis vs. design
  • Analysis: determine output given input and sequential network
  • Design: input and output are known; need to determine makeup of sequential network
  • General approach:
    • construct state transition table and transition diagram
    • determine output stream for given input stream
excitation table
Excitation table
  • The excitation table is a design tool for constructing circuits from a given type of flip-flop
  • Given the desired transition from Q(t) to Q(t +1), what inputs are necessary to make the transition happen?
characteristic table vs excitation table for sr flip flop
Characteristic table vs. Excitation table for SR flip flop
  • Tells what next state is, given current input and current state
  • Tells what current input must be given current state
sequential analysis
Sequential analysis
  • Step 1: List all possible combinations of current state and current input in an analysis table
  • Step 2: For each combination, compute the output and the current inputs to the state registers
  • Step 3: From the characteristic table, determine the next state and construct the state transition table and diagram
example problem
Example problem
  • State registers: FFA & FFB (T flip flops)
  • Combinational circuit
    • inputs:
      • X1 AND B (TA)
      • X2 OR A (TB)
      • TA & TB are inputs to FFA & FFB
    • output:
      • B’ AND X1 (Y)
example problem21
Example problem
  • 2 flip flops, so 4 possible states:
  • 2 inputs, so 4 possible input combinations:
example problem22
Example problem
  • Given a state (AB) and an input (X1X2):
    • what is output?
    • what will be the state after CP?
  • 16 possible answers, as shown on next slide
analysis table for sample problem circuit
Analysis table for sample problem circuit
  • 1st 4 columns list possible combinations of initial state & initial input
  • By the logic diagram, we know:
    • Y(t)=X1(t) AND B’(t)
    • TA(t)=X1(t) AND B(t)
    • TB(t)=X2(t) OR A(t)
  • Compute next 3 columns given above
  • Compute last 2 from:
    • characteristic table for T flip flop
    • initial state of flip flop
    • flip flop’s initial input
state transition table
State transition table
  • Table shows simple rearrangement of selected columns from table on previous slide
  • For given initial state A(t)B(t) and input X1(t)X2(t), lists next state (A+1)(t)(B+1)(t) and initial output Y(t)
  • States listed as ordered pairs – next state followed by initial output
state transition diagram
State transition diagram
  • Easier to visualize circuit behavior
  • Transitions listed as ordered pairs of input followed by initial output, with slash separator
asynchronous inputs
Asynchronous inputs
  • An asynchronous input changes state of a flip-flop immediately without regard to CP
    • Preset sets Q to 1
    • Clear clears Q to 0
  • Used to initialize the state of a machine
  • Normal operation: both lines 0
sequential design
Sequential design
  • Given the state transition diagram, the output, and the type of flip-flop to be used, design the combinational circuit
  • Any unused input combinations or unused states are don’t care conditions
  • 2n states are possible with n flip-flops
design steps
Design steps
  • Step 1: In a design table, list the initial state, input, and output, and from the transition diagram list the next state
  • Step 2: Use the excitation table for the given type of flip-flop to determine the input required for the state registers
  • Step 3: Use Karnaugh maps to design a minimized two-level circuit for each flip-flop input
sequential design k maps
Sequential design & K-maps
  • Each flip flop in the problem can be considered a function of four variables:
    • initial state (AB)
    • input (X1X2)
  • To design the combinational circuit we need a 4-variable K-map for each flip flop input
k maps for sample problem
K-maps for sample problem
  • Figures a and b below show K-maps for S & R inputs to FFA
    • Row values are AB, columns are X1X2
    • X1X2 = 00 is a don’t care condition for both inputs, so first column of both tables is X
k maps for sample problem33
K-maps for sample problem
  • Figures c and d show inputs to FFB
  • Note that we can take advantage of don’t care conditions to minimize circuit
another look at the register
Another look at the register
  • Basic building block of instruction set architecture
    • array of D flip flops; each is bit in register
    • common clock line connected to all flip flops; # of flip flops doesn’t affect speed of load operation because all receive clock signal simultaneously
  • Conceptually, main memory is just a big array of registers
  • Input: address lines, control lines, data lines
  • Data lines are bidirectional (output also)
  • Control signals:
    • CS: Chip select, to enable or select the memory chip
    • WE: Write enable, to write or store a memory word to the chip
    • OE: Output enable, to enable the output buffer to read a word from the chip
memory chips
Memory chips

Storage capacity of each is identical (512 bits); left uses 8-bit word, right uses 1

Generally, chip with 2n words has n address lines

memory access
Memory access
  • To store a word (memory write)
    • Select chip by setting CS to 1
    • Put data and address on the bus and set WE to 1
  • To retrieve a word (memory read)
    • Select chip by setting CS to 1
    • Put address on the bus, set OE to 1, and read the data on the bus
4 x 2 memory chip
4 x 2 memory chip
  • 2 address lines (A0, A1) & 2 data lines (D0, D1)
  • Stores 4 2-bit words
    • each bit is D flip flop
  • Address lines drive 2 x 4 decoder
    • 1 output is 1, other 3 0
    • line with 1 signal selects row of D flip flops that make up word accessed by chip
closer look
Closer look

Diagram below shows implementation of “Read enable” box

Alphabet soup:

WE: write enable

CS: chip select

OE: output enable

MMV: monostable multivibrator (CP)

read enable
Read Enable
  • Three normal modes:
    • CS=0 (chip not selected)
    • CS=1, WE=1, OE=0

(chip selected for write)

    • CS=1, WE=0, OE=1

(chip selected for read)

  • WE & OE not permitted to be 1 at same time
memory types volatile
Memory types: volatile
  • SRAM: Static random access memory
    • most closely resembles model we’ve seen
    • advantage: fast
    • disadvantage: large – several transistors required for each bit cell
  • DRAM: Dynamic RAM
    • overcomes size problem of SRAM: one transistor, one capacitor per cell
    • advantage: high capacity
    • disadvantage: relatively slow because requires refresh operation
memory types non volatile
Memory types: non-volatile
  • ROM: Read-only memory
    • Simplest type, ROM, is prewritten to spec by manufacturer – can’t be overwritten
    • PROM: Programmable ROM: user can write once (by blowing embedded fuses) – can’t be overwritten
    • EPROM: Erasable PROM: can be wiped out & reprogrammed (requires removal from computer)
memory types non volatile45
Memory types: non-volatile
  • EEPROM: Electrically erasable PROM
    • Like EPROM, but doesn’t require removal to reprogram
    • Can reprogram individual cell (doesn’t have to be whole chip)
  • Flash memory: A type of EEPROM
    • flash card is array of flash chips
    • flash drive has interface circuitry to mimic hard drive