George Mason University ECE 545 – Introduction to VHDL Data types Timing in VHDL ECE 545 Lecture 13

ECE 545 – Introduction to VHDL George Mason University

Data types Timing in VHDL

ECE 545Lecture 13

ECE 545 – Introduction to VHDL 2

Sources & Required Reading

• Peter Ashenden, The Designer’s Guide to VHDL,

Chapter 2

Scalar Data Types and OperationsChapter 4

Composite Data Types and Operations


VHDL as a Strongly Typed Language


Notion of type

• Type defines a set of values and a set of applicable operations

• Declaration of a type determines which values can be stored in an object (signal, variable, constant) of a given type

• Every object can only assume values of its nominated type

• Each operation (e.g., and, +, *) includes the types of values to which the operation may be applied, and the type of the result

• The goal of strong typing is a detection of errors at an early stage of the design process


Example of strong typing

architecture incorrect of example1 is

type apples is range 0 to 100;type oranges is range 0 to 100;

signal apple1: apples;signal orange1: oranges;

begin apple1 <= orange1;end incorrect;


Type Classification


Classification of data types


Integer Types


Integer type

Name: integer

Status: predefined

Contents: all integer numbers representable on a particular host computer, but at least numbers in the range

–(231-1) .. 231-1


User defined integer types - Examples

type day_of_month is range 0 to 31;

type year is range 0 to 2100;

type set_index_range is range 999 downto 100;

constant number_of_bits: integer :=32;

type bit_index is range 0 to number_of_bits-1;

Values of bounds can be expressions, but

need to be known when the model is analyzed.


Enumeration Types


Predefined enumeration types (1)

boolean (true, false)

bit (‘0’, ‘1’)

character VHDL-87:128 7-bit ASCII characters

VHDL-93:256 ISO 8859 Latin-1 8-bit characters


Predefined enumeration types (2)

severity_level (note, warning, error, failure)

Predefined in VHDL-93 only:

file_open_kind(read_mode, write_mode, append_mode)

file_open_status (open_ok, status_error, name_error, mode_error)


User-defined enumeration types - Examples

type state is (S0, S1);

type alu_function is (disable, pass, add, subtract,

multiply, divide);

type octal_digit is (‘0’, ‘1’, ‘2’, ‘3’, ‘4’, ‘5’, ‘6’, ‘7’);

type mixed is (lf, cr, ht, ‘-’, ‘/‘, ‘\’);

Each value in an enumeration type must be either

an identifier or a character literal


Floating-Point Types


Floating point types

• Used to represent real numbers• Numbers are represented using a significand

(mantissa) part and an exponent part• Conform to the IEEE standard 754 or 854

Minimum size of representation that must besupported by the implementation of the VHDLstandard:

VHDL-2001: 64-bit representationVHDL-87, VHDL-93: 32-bit representation


Real literals - examples

23.1 23.1

46E5 46 105

1E+12 1 1012

1.234E09 1.234 109

34.0e-08 34.0 10-8

2#0.101#E5 0.1012 25 =(2-1+2-3) 25

8#0.4#E-6 0.48 8-6 = (4 8-1) 8-6

16#0.a5#E-8 0.a516 16-8 =(1016-1+516-2) 16-8


The ANSI/IEEE standard floating-point number representation formats


User-defined floating-point types - Examples

type input_level is range -10.0 to +10.0

type probability is range 0.0 to 1.0;

constant max_output: real := 1.0E6;constant min_output: real := 1.0E-6;type output_range is max_output downto min_output;


Attributes of Scalar Types


Attributes of all scalar types

T’left first (leftmost) value in TT’right last (rightmost) value in TT’low least value in TT’high greatest value in T

Not available in VHDL-87:T’ascending

true if T is an ascending range, false otherwise

T’image(x) a string representing the value of xT’value(s) the value in T that is represented by s


type index_range is range 21 downto 11;

index_range’left = 21index_range’right = 11index_range’low = 11index_range’high = 21index_range’ascending = falseindex_range’image(14) = “14”index_range’value(“20”) = 20

Attributes of all scalar types - examples


Attributes of discrete types

T’pos(x) position number of x in T

T’val(n) value in T at position n

T’succ(x) value in T at position one greater than position of x

T’pred(x) value in T at position one less than position of x

T’leftof(x) value in T at position one to the left of x

T’rightof(x) value in T at position one to the right of x


type logic_level is (unknown, low, undriven, high);

logic_level’pos(unknown)= 0logic_level’val(3) = highlogic_level’succ(unknown) = lowlogic_level’pred(undriven) = lowlogic_level’leftof(unknown) errorlogic_level’rightof(undriven) = high

Attributes of discrete types - examples


Subtypes


Subtype

• Defines a subset of a base type values• A condition that is used to determine which

values are included in the subtype is called a constraint

• All operations that are applicable to the base type also apply to any of its subtypes

• Base type and subtype can be mixed in the operations, but the result must belong to the subtype, otherwise an error is generated.


Predefined subtypes

natural integers 0

positive integers > 0

Not predefined in VHDL-87:

delay_lengthtime 0


User-defined subtypes - Examples

subtype bit_index is integer range 31 downto 0;

subtype input_range is real range 1.0E-9 to 1.0E+12;


Operators


Operators (1)


Operators (2)


Operators (3)


Operator Overloading


Operator overloading

• Operator overloading allows different argument types for a given operation (function)

• The VHDL tools resolve which of these function to select based on the types of the inputs

• This selection is transparent to the user as long as the function has been defined for the given argument types.


Different declarations for the same operator - Example

Declarations in the package ieee.std_logic_unsigned:

function “+” ( L: std_logic_vector; R:std_logic_vector)

return std_logic_vector;

function “+” ( L: std_logic_vector; R: integer)


function “+” ( L: std_logic_vector; R:std_logic)



Different declarations for the same operator - Example

signal count: std_logic_vector(7 downto 0);

You can use:

count <= count + “0000_0001”;

or

count <= count + 1;

or

count <= count + ‘1’;


Specifying time in VHDL


Physical data types

Types representing physical quantities, such as time, voltage, capacitance, etc. are referred in VHDL as physical data types.

TIME is the only predefined physical data type.

Value of the physical data type is called a physical literal.


Time values (physical literals) - Examples

7 ns

1 min

min

10.65 us

10.65 fs

Unit of time

(dimension)

SpaceNumeric value


TIME values

Numeric value can be an integer or

a floating point number.

Numeric value is optional. If not given, 1 is

implied.

Numeric value and dimension MUST be

separated by a space.


Units of time

Unit Definition

Base Unit

fs femtoseconds (10-15 seconds)

Derived Units

ps picoseconds (10-12 seconds)

ns nanoseconds (10-9 seconds)

us microseconds (10-6 seconds)

ms miliseconds (10-3 seconds)

sec seconds

min minutes (60 seconds)

hr hours (3600 seconds)


Values of the type TIME

Value of a physical literal is defined in terms

of integral multiples of the base unit, e.g.

10.65 us = 10,650,000,000 fs

10.65 fs = 10 fs

Smallest available resolution in VHDL is 1 fs.

Smallest available resolution in simulation can be

set using a simulator command or parameter.


Arithmetic operations on values of the type TIME

Examples:

7 ns + 10 ns = 17 ns

1.2 ns – 12.6 ps = 1187400 fs

5 ns * 4.3 = 21.5 ns

20 ns / 5ns = 4


Propagation delay in VHDL


Propagation delay in VHDL - Example

entity MAJORITY isport (A_IN, B_IN, C_IN : in STD_LOGIC; Z_OUT : out STD_LOGIC);end MAJORITY;

architecture DATA_FLOW of MAJORITY isbeginZ_OUT <= (not A_IN and B_IN and C_IN) or

(A_IN and not B_IN and C_IN) or(A_IN and B_IN and not C_IN) or(A_IN and B_IN and C_IN) after 20 ns;

end DATA_FLOW;


Propagation delay - Example


MLU: Block Diagram

B

A

NEG_A

NEG_B

IN0

IN1

IN2

IN3 OUTPUT

SEL1

SEL0

MUX_4_1

L0L1

NEG_Y

Y

Y1

A1

B1

MUX_0

MUX_1

MUX_2

MUX_3


MLU - Architecture Body – Example 1begin

A1<= not A after 6 ns when (NEG_A='1') elseA after 5 ns;

B1<= not B after 6 ns when (NEG_B='1') else B after 5 ns;

Y <= not Y1 after 6 ns when (NEG_Y='1') elseY1 after 5 ns;

MUX_0 <= A1 and B1 after 3 ns;MUX_1 <= A1 or B1 after 3 ns;MUX_2 <= A1 xor B1 after 4 ns;MUX_3 <= A1 xnor B1 after 5 ns;

L<=L1 & L0;

with (L) select Y1 <= MUX_0 after 7 ns when "00",

MUX_1 after 6 ns when "01", MUX_2 after 8 ns when "10",

MUX_3 after 7 ns when others;

end MLU_DATAFLOW;


MLU - Architecture Body – Example 2begin

A1<= not A after MUX2_delay when (NEG_A='1') elseA after MUX_2_delay;

B1<= not B after MUX2_delay when (NEG_B='1') else B after MUX2_delay;

Y <= not Y1 after MUX2_delay when (NEG_Y='1') elseY1 after MUX2_delay;

MUX_0 <= A1 and B1 after GATE_delay;MUX_1 <= A1 or B1 after GATE_delay;MUX_2 <= A1 xor B1 after XOR_delay;MUX_3 <= A1 xnor B1 after XOR_delay;

L<=L1 & L0;

with (L) select Y1 <= MUX_0 after MUX4_delay when "00",

MUX_1 after MUX4_delay when "01", MUX_2 after MUX4_delay when "10",

MUX_3 after MUX4_delay when others;

end MLU_DATAFLOW;


Delay constants

constant MUX2_delay: time := 5 ns;

constant GATE_delay : time := 3 ns;

constant XOR_delay : time := 4 ns;

constant MUX4_delay: time := 7 ns;

Can be defined in the declarative portion

of the architecture or in the package


Inertial delay model


Inertial delay model

Short pulses (spikes) are not passed to the

outputs of logic gates due to the inertia of

physical systems.

Logic gates behave like low pass filters and

effectively filter out high frequency input

changes as if they never occurred.


Inertial delay model - Example

SIG_OUT <= not SIG_IN after 7 ns


VHDL-87 Inertial delay model

Any input signal change that does not persist

for at least a propagation delay of the device

is not reflected at the output.

inertial delay (pulse rejection limit) = propagation delay


VHDL-93 Enhanced inertial delay model

VHDL-93 allows the inertial delay model to be declaredexplicitly as well as implicitly.

Explicitly:Z_OUT <= inertial (not A_IN and B_IN and C_IN) or


Implicitly:Z_OUT <= (not A_IN and B_IN and C_IN) or



VHDL-93 Enhanced inertial delay model

VHDL-93 allows inertial delay, also called

a pulse rejection limit, to be different from the

propagation delay.

SIG_OUT <= reject 5 ns inertial not SIG_IN after 7 ns;


Transport delay model

With a transport delay model, all input signal

changes are reflected at the output, regardless of

how long the signal changes persist.

Transport delay model must be declared explicitly using the

keyword transport.

Inertial delay model is a default delay model because it

reflects better the actual behavior of logic components.

Transport delay model is used for high-level modeling.


Transport delay model - Example

SIG_OUT <= transport not SIG_IN after 7 ns


Other delay models

Rise and Fall delays

- a different delay for a transition 0→1

and a transition 1→0


Event-driven simulation


Event list as a linked list structure

time

signalnew value

List of events scheduled

to occur at time tq


Event list as an array – Timing wheel

time

signalnew value

List of events scheduled

to occur at time tc


Notation

(i, vi’) – an entry of the event list associated with the time t indicating that at the time t the value of signal i is scheduled to be set to vi’

v(i) – current value at the output of gate i

d(i) – nominal delay of gate i


Top-level algorithm

while (event list not empty)

begin

t = next time in list

process entries for time t

end


Process entries for time t - Basic version

Activated = Ø /* set of activated gates = empty set */

For every entry (i, vi’) pending at the current time t if vi’ ≠ v(i) then

begin /* it is indeed an event */v(i) = vi’ /* update value of signal i */for every j on the fanout list of i

beginupdate input values of jadd j to Activated

endend

For every j Activatedbegin

vj’ = evaluate(j)schedule (j, vj’) for time t+d(j)

end


Event-driven simulation - example

(z, 1)

8 10 12

(z, 0) (z, 0)

time


Notation

lsv(j) – last scheduled value of j

lst(j) – last scheduled time of j = time of the last event scheduled for signal j


Delta delay


Delta delay

A propagation delay of 0 time units is

equivalent to omitting the after clause and is

called a delta delay.

Used for functional simulation.


Two-dimensional aspect of time


Top-level algorithm

while (event list not empty)

begin

t = next time in list

process entries for time t

end

If next time in list

= previous time

then the previous

iteration of the

loop has advanced

time by one

delta delay


Transactions vs. Events


Transaction vs. Event

T5 = T1+20 ns

Z_OUT transactions

Z_OUT events

(‘0’, T1 + 20 ns) (‘1’, T2 + 20 ns) (‘0’, T3 + 20 ns)

(‘1’, T2 + 20 ns) (‘0’, T3 + 20 ns)


Properties of signals

Signals represent a time-ordered list of valuesdenoting past, present and future values.

This time history of a signal is called a waveform.

A value/time pair (v, t) is called a transaction.

If a transaction changes value of a signal, it iscalled an event.


Signal Attributes


Signal attributes (1)

S’transaction - a signal of type bit that changes value from ‘0’ to ‘1’ or vice versa each time there is a transaction on S.

S’event - True if there is an event on S in the current simulation cycle, false otherwise.

S’active – True if there is a transaction on S in a given simulation cycle, false otherwise.



S’last_event - The time interval since the last event on S.

S’last_active - The time interval since the last transaction on S.

S’last_value – The value of S just before the last event on S.



S’delayed(T) - A signal that takes on the same value as S, but is delayed by time T.

S’stable(T) - A Boolean signal that is true if there has been no event on S in the time interval T up to the current time, otherwise false.

S’quiet(T) – A Boolean signal that is true if there has been no transaction on S in the time interval T up to the current time, otherwise false.


Detecting setup time violation

if clk’event and clk=‘1’ then

assert d’last_event >= setup_time

report “Setup time violation”


algorithmic

Design level

register-transfer

gate

transistor

layout

devices

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Introduction to VHDL

DigitalIntegratedCircuitsMixed

Signals VLSI

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ECE 699

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ECE684 MOS Device Electronics

ECE 584 SemiconductorDevice Fundamentals

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George Mason University ECE 545 – Introduction to VHDL Data types Timing in VHDL ECE 545 Lecture 13