Ch 5 . Logic Design with MSI Components

Ch 5.Logic Design with MSI

Components

Csci 2021 Srping02 2

VHDL The U.S. Department of Defense (DoD) supported the development of VHDL

(VHSIC hardware description language) as part of the VHSIC (very high-speed IC) program in the early 1980s. The companies in the VHSIC program found they needed something more than schematic entry to describe large ASICs, and proposed the creation of a hardware description language. VHDL was then handed over to the Institute of Electrical and Electronics Engineers (IEEE) in order to develop and approve the IEEE Standard 1076-1987. 1 As part of its standardization process the DoD has specified the use of VHDL as the documentation, simulation, and verification medium for ASICs (MIL-STD-454). Partly for this reason VHDL has gained rapid acceptance, initially for description and documentation, and then for design entry, simulation, and synthesis as well.

The first revision of the 1076 standard was approved in 1993. References to the VHDL Language Reference Manual (LRM) in this chapter--[VHDL 87LRM2.1, 93LRM2.2] for example--point to the 1987 and 1993 versions of the LRM [IEEE, 1076-1987 and 1076-1993]. The prefixes 87 and 93 are omitted if the references are the same in both editions. Technically 1076-1987 (known as VHDL-87) is now obsolete and replaced by 1076-1993 (known as VHDL-93). Except for code that is marked 'VHDL-93 only' the examples in this chapter can be analyzed (the VHDL word for "compiled") and simulated using both VHDL-87 and VHDL-93 systems.






Logic Gates and Symbols

ab F

ab F a F

F = a b F = a + b F = !a

And Or Not

A B F

0 0 0

0 1 0

1 0 0

1 1 1

A B F

0 0 0

0 1 1

1 0 1

1 1 1

A F

0 1

1 0





Logic Equation Representation: Sum-of-Products (SOP)

SOP form:

A collection of ANDed variables are Ored together.

Example:

A B F0 0 10 1 01 0 01 1 1

F = !A!B + AB

Also called XNOR


Example of SOPExample: A three-input majority function

The function is true when more than half of its inputs are true

F =?

A B C F

0 0 0 0

0 0 1 0

0 1 0 0

0 1 1 1

1 0 0 0

1 0 1 1

1 1 0 1

1 1 1 1


Example of SOPExample: A three-input majority function

The function is true when more than half of its inputs are true

F =!ABC+A!BC+AB!C+ABC

A B C F

0 0 0 0

0 0 1 0

0 1 0 0

0 1 1 1

1 0 0 0

1 0 1 1

1 1 0 1

1 1 1 1



Digital Components• High level digital designs are usually made using collections

of logic gates. Such collection of gates are referred as components.• Multiplexer and Decoder are commonly used digital

components.• Levels of integration

SSI (Small Scale Integration) 10-100 components per chipMSI (Medium Scale Integration) 100-1,000 components per chipLSI (Large Scale Integration) 1000-10,000 components per chipVLSI (Very Large Scale Integration) – HigherULSI (Ultra Large Scale Integration) – Higher, higher!




Example: using MUX for Majority


An 8-bit multiplexer

entity Mux8 is

generic (TPD : TIME := 1 ns);

port (A, B : in BIT_VECTOR (7 downto 0);

Sel : in BIT := '0'; Y : out BIT_VECTOR (7 downto 0));

end;

architecture Behave of Mux8 is

Begin

Y <= A after TPD when Sel = '1' else B after TPD;

end; Eight 2:1 MUXs withsingle select input.

Timing: TPD (input to Y) = 1 ns




Example: Using Decoder for Majority






Carry-In A B Sum Carry-out

0 0 0 0 0

0 0 1 1 0

0 1 0 1 0

0 1 1 0 1

1 0 0 1 0

1 0 1 0 1

1 1 0 0 1

1 1 1 1 1

Sum = A’BC’ + AB’C’ + A’B’C + ABCCarry-out = ABC’ + A’BC + AB’C + ABC


A full adder

Entity Full_Adder is generic (TS : TIME := 0.11 ns; TC : TIME := 0.1 ns); port (X, Y, Cin: in BIT; Cout, Sum: out BIT); end Full_Adder;

architecture Behave of Full_Adder is begin Sum <= X xor Y xor Cin after TS; Cout <= (X and Y) or (X and Cin) or (Y and Cin) after TC; end; Timing:TS (Input to Sum) = 0.1 1 nsTC (Input to Cout) = 0.1 ns


An 8-bit ripple-carry adder entity Adder8 is port (A, B: in BIT_VECTOR(7 downto 0); Cin: in BIT; Cout: out BIT; Sum: out BIT_VECTOR(7 downto 0));end Adder8; architecture Structure of Adder8 is component Full_Adder port (X, Y, Cin: in BIT; Cout, Sum: out BIT); end component; signal C: BIT_VECTOR(7 downto 0); begin Stages: for i in 7 downto 0 generate LowBit: if i = 0 generate FA:Full_Adder port map (A(0),B(0),Cin,C(0),Sum(0)); end generate; OtherBits: if i /= 0 generate FA:Full_Adder port map (A(i),B(i),C(i-1),C(i),Sum(i)); end generate; end generate; Cout <= C(7); end;


The single input lineinto each AND gaterepresents 6 input lines

The single input line into each OR gate represents 8 lines

Darkened circles are placedat crosspoints to indicate connections are made





When A,B,C all changed from 0 to 1, there willBe a glitch.


Flip-Flop

• A Flip-flop is an arrangement of logic gates that maintains a stable output even after the inputs are made inactive.

• A flip flop can be used to store a single bit of information.

• A S-R flip flop holds a single bit of information and serve as an elementary memory cell.

• In order to achieve synchronization in a controlled fashion, a clock signal is provided. Every state-dependent circuit synchronizes itself by accepting inputs only at discrete times.










Truth Table for Mod-4 Counter

RESET S1 S0 S1/S0 Q1/Q0

0 0 0 0/1 0/1

0 0 1 1/0 1/0

0 1 0 1/1 1/1

0 1 1 0/0 0/0

1 0 0 0/0 0/0

1 0 1 0/0 0/0

1 1 0 0/0 0/0

1 1 1 0/0 0/0

Note that S1/S0 are identical to Q1/Q0






Finite State Machine

4X5 PLA

Q DS0

Q DS1

X1

X0

Z2

Z1Z0

CLK


Truth Table for Vending Machine S1 S0 X1 X0 S1 S0 Z0 Z1 Z2

0 0 0 0 0 1 0 0 0

0 0 0 1 1 0 0 0 0

0 0 1 0 0 0 1 1 0

0 1 0 0 1 0 0 0 0

0 1 0 1 1 1 0 0 0

0 1 1 0 0 0 1 0 1

1 0 0 0 1 1 0 0 0

1 0 0 1 0 0 1 0 0

1 0 1 0 0 0 1 1 1

1 1 0 0 0 0 1 0 0

1 1 0 1 0 0 1 1 0

1 1 1 0 0 1 1 1 1


Example

• Assume our vending machine takes only nickels and dimes.

• The machine vends items for 15 cents.

• What is the state transition diagram?


Example

• Assume our vending machine takes only nickels and dimes.

• The machine vends items for 15 cents.

• What is the state transition diagram?

A0 cent

B5 cent

C10 cent

N/00

N/00

N/10

D/00

D/10

D/11

N/D: Nickel or Dime0/1: dispense or not 0/1: return nickel or not

Documents

Ch 5 . Logic Design with MSI Components