Sequential circuits part 2: implementation, analysis & design All illustrations  , 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: – JK (don’t know why it’s called that) – D (data) – T (toggle)

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

Implementation of JK flip flop

JK flip flop implementation If JK = 00, SR = 00 because of AND – so SR won’t change state when clocked

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 implementation If JK = 01, final state is Q=0 (analogous to JK=10)

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: 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 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 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 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 Identical to JK, with J=K

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 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 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 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 Tells what next state is, given current input and current state Tells what current input must be given current state

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 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 problem 2 flip flops, so 4 possible states: AB inputs, so 4 possible input combinations: X1X

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 1 st 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 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 Easier to visualize circuit behavior Transitions listed as ordered pairs of input followed by initial output, with slash separator

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 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 2 n states are possible with n flip-flops

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

Sample problem

Design table for sample problem

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 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 problem Figures c and d show inputs to FFB Note that we can take advantage of don’t care conditions to minimize circuit

Resulting circuit with original spec

K-map & circuit for output Y

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

Memory 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 Storage capacity of each is identical (512 bits); left uses 8-bit word, right uses 1 Generally, chip with 2 n words has n address lines

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 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 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 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 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 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-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