Overview• Finite State Machines

- Sequential circuits with inputs and outputs

• State Diagrams

- An abstraction tool to visualize and analyze sequential circuits

• Internal Memory

- Random Access Memory (RAM)

- Volatile – values lost on power loss

- Static RAM (SRAM)- Dynamic RAM (DRAM)

- Read Only Memory (ROM)

Combinational vs. Sequential Logic

• There are two types of “combination” locks

4 1 8 4

30

15

5

1020

25

Combinational:Success depends only onthe values, not the order in which they are set.

Sequential:Success depends onthe sequence of values(e.g, R-13, L-22, R-3).

Combinational vs. Sequential Circuits

• Combinational Circuit– always gives the same output for a given set of inputs

• example: adder always generates sum and carry,regardless of previous inputs

• Sequential Circuit– has memory - “stores” information,– output depends on stored information (state) plus input

• so a given input might produce different outputs,depending on the stored information

State Machine

• A type of sequential circuit– Combines combinational logic with storage– “Remembers” state, and changes output (and

state) based on inputs and current state

State Machine

CombinationalLogic Circuit

StorageElements

Inputs Outputs

State

• The state of a system is a snapshot of all the relevant elements of the system at the moment the snapshot is taken.

•Examples:– The state of a basketball game can be represented by

the scoreboard.(Number of points, time remaining, possession, etc.)

– The state of a tic-tac-toe game can be represented bythe placement of X’s and O’s on the board.

State of Sequential Lock

Our lock example has four different states, labelled A-D:

A: The lock is not open,and no relevant operations have been performed.

B: The lock is not open,and the user has completed the R-13 operation.

C: The lock is not open,and the user has completed R-13, followed by L-

22.

D: The lock is open.

Finite State Machine

• A description of a system with the following components:

1. A finite number of states2. A finite number of external inputs3. A finite number of external outputs4. An explicit specification of all state transitions5. An explicit specification of what determines each external

output value

• Often described by a state diagram. - The set of all possible states.

- Inputs that trigger state transitions.- Outputs associated with each state (or with each

transition).

State Diagram

• Shows states (e.g. A), actions (e.g. B) that cause a transition between states, and the outputs.

Open

Locked Locked

Locked

The Clock• Frequently, a clock circuit triggers transition fromone state to the next.

• At the beginning of each clock cycle, the state machine makes a transition, based on the current state and the external inputs (Synchronous).

– Not always required. In lock example, the input itself triggers a transition (Asynchronous).

“1”

“0”

timeOneCycle

Implementing a Finite State Machine

• Combinational logic– Determine outputs at each state. – Determine next state.

• Storage elements– Maintain state representation.

State Machine

CombinationalLogic Circuit

StorageElements

Inputs Outputs

Clock

Storage• Each D flipflop stores one state bit.

• The number of storage elements (flipflops) neededis determined by the number of states(and the representation of each state).

• Examples:– Sequential lock

•Four states – two bits– Basketball scoreboard

•7 bits for each score digit, 5 bits for minutes, 6 bits for seconds,1 bit for possession arrow, 1 bit for half, …

Complete Example – Traffic Sign

• Design a “blinking” traffic sign which exhibits this behavior:

State 1) No lights on State 2) 1 & 2 on State 3) 1, 2, 3, & 4 on State 4) 1, 2, 3, 4, & 5 on State 1) No lights on

.

.( - Repeat as long as operate

switch is turned on. - The system is in state 1 when the operate switch is off)

DANGERMOVERIGHT

1

2

3

4

5

Traffic Sign State Diagram

State bit S1 State bit S0

Switch onSwitch off

Outputs

State Transitions occur on each clock cycle.

Traffic Sign Truth Tables

Outputs(depend only on state:

S1S0)

S1 S0 Z Y X

0 0 0 0 0

0 1 1 0 0

1 0 1 1 0

1 1 1 1 1

Lights 1 and 2

Lights 3 and 4

Light 5

Next State: S1’ S0’(depend on state and

input)

In S1 S0 S1 S0

0 X X 0 0

1 0 0 0 1

1 0 1 1 0

1 1 0 1 1

1 1 1 0 0

Switch

Whenever In=0, next state is 00.

Traffic Sign Combinational Logic

Edge Triggered D flipflops

Another Example of a State Machine

Digital Computer “States”:

1. Fetch Instruction

1. Fetch Operand(s)

1. Execute Operation

1. Store Result

1. Check for Interrupt

2. Go to 1.

Computer

Memory

Computer Memory Hierarchy

Main Memory

• Address Space– The number of uniquely addressable memory locations

• Addressability– The number of bits stored at an addressable location

• Unit of Transfer– The number of bits transferred in a memory read or

write {could be the “addressability” or a multiple of it, i.e. the addressability could be

– an 8 bit byte, or – a 32 bit word (4 bytes) }

Basic Types of Memory

Two basic kinds of RAM (Random Access Memory)

• Static RAM (SRAM)– fast, maintains data as long as power applied

• Dynamic RAM (DRAM)– slower but denser, bit storage decays – must be

periodically refreshed. Refreshing interferes with regularity of execution of instruction stream.

Also, non-volatile memories: ROM, PROM, flash, …

Memory Map

00 00000000

01 01010101

02 11001010

03 00011001

. .

. .

FF 11001100

What is the Address Space of this memory?

What is the Word Length of this memory?

What is the Unit of Transfer of this memory?

Memory Organization

• What would a 1 word by 1 bit memory look like?– How could data be stored in it?– How could data be read from it?

• What would a 1 word by 2 bit memory look like?– How could data be stored in it ?– How could data be read from it ?

• What would a 2 word by 1 bit memory look like?– How could data be stored in it ?– How could data be read from it ?

22 x 3 Memory Organization

addressdecoder

word select word WEaddress

writeenable

input bits

output bits

22 x 3 Memory – 1 Decoder, 3 Multiplexors

22 x 3 Memory – Read of Word at Address 11

Memory Design – 1K x 4

A[09:00] D[03:00]

Addr Block Select

Memory Design – 1K x 8

A[09:00] D[07:04]

A[09:00] D[03:00]

Addr Block Select =>

Addr Block Select =>

D[07:04] D[03:00]

Memory Design - 2k x 8

D[07:04] D[03:00]

Block 01

Block 00

Memory Design - 4k x 8

D[07:04] D[03:00]

Block 11

Block 10

Block 01

Block 00