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- Table des matières
- MARQUE LIVRES
Noté. / 5. Basé sur avis des utilisateurs
How to Write Synthesizable VHDL
VHDL Reference Manual 3-19
Latches
The following three examples each describe a level sensitive latch with
an and function connected to its input. In all three of these examples
the signal y retains its current value unless the clock input (clk) is
high.
Example 1:
This example uses a process statement and conditional (if)
statement. The sensitivity list contains the clock input and two data
inputs because when clk is high the output y changes asynchronously
with any change in a or b:
process (clk,a,b)
begin
if clk='1' then
y <= a and b;
end if;
end process;
Example 2:
This example uses a procedure statement in combination with a
concurrent procedure call. In this example the procedure is called
twice to generate two latches from the declared procedure:
architecture dataflow of latch is
procedure my_latch(signal clk,a,b: in Boolean;
signal y : out Boolean)
begin
if clk='1' then
y <= a and b;
end if;
end;
begin
latch_1: my_latch (clock,input1,input2,outputA);
label_2: my_latch (clock,input1,input2,outputB);
end dataflow;
Example 3:
This example uses a concurrent conditional assignment to describe a
latch function for y. Note that y is used as an input to the conditional
statement as well as being used as the output:
architecture dataflow of latch is
begin
y <= a and b when clk else y;
end dataflow;
Flip-flops
The following four examples describe an edge sensitive flip-flop with
an AND function connected to its input. In all these cases the signal y
retains its current value unless the clock is changing.
- VHDL Reference 1
- Table of Contents 3
- 1. Introduction 7
- Introduction 8
- 1-2 VHDL Reference Manual 8
- 2. Language Structure 9
- Architecture 13
- Configuration 13
- Statements 14
- Data Objects 16
- Constants 17
- Data Types 19
- Numeric Types 20
- Other Types 21
- Enumerated Types 21
- Types and Logic Synthesis 23
- Type Conversions 23
- Operators 24
- VHDL Attributes 25
- variable info : my_array; 26
- signal clock : Boolean; 26
- 3. How to Write 27
- Constants and Types 29
- Logical Operators 29
- Relational Operators 31
- Arithmetic Operators 32
- Describing Conditional Logic 34
- If Statement 35
- Case Statement 36
- Describing Replicated Logic 37
- Loop Statements 38
- Generate Statements 39
- Describing Sequential Logic 41
- Wait Statement 44
- Flip-flops 45
- Gated Clocks and Clock Enable 47
- Synchronous Set/Reset 48
- Asynchronous Set/Reset 48
- Asynchronous Reset/Preset 49
- Template State Machine 51
- Feedback Mechanisms 52
- Types of State Machines 54
- Moore Machine 54
- Mealy Machine 56
- 4. How to Control the 59
- Synthesis of Enumerated Types 60
- Enum_encoding attribute 60
- “One hot” Enumeration 62
- Describing Output Enables 63
- Controlling Output Inversion 64
- VHDL Reference Manual 4-7 65
- Controlling Feedback Paths 66
- Selecting a Base Data Type 68
- Synthesis of Don't Cares 71
- Pinnum Attribute 72
- Property Attribute 73
- Macrocell Attribute 74
- Critical Attribute 75
- Enum_encoding Attribute 76
- 5. VHDL Datapath Synthesis 77
- How Inferencing Works 78
- Inferencing Details 83
- Instantiation Details 85
- 6. How to Manage VHDL 91
- Components 92
- Components And Synthesis 93
- Configurations 94
- Using Packages 95
- Package Visibility Rules 96
- Design Libraries 96
- Using Design Libraries 98
- Using Schematics With VHDL 99
- 6-12 VHDL Reference Manual 102
- A. VHDL Quick Reference 103
- VHDL Syntax Basics 104
- Sequential Statements 105
- Subprograms 106
- Concurrent Statements 107
- VHDL Quick Reference 108
- A-6 VHDL Reference Manual 108
- Library Units 109
- Attributes 110
- B. Limitations 111
- Ignored Constructs 112
- C. VHDL for the ABEL-HDL 113
- Pin and Node Numbers 114
- Combinational Logic 115
- Registers 116
- Avoiding Unwanted Latches 119
- State Machines 121
- Design I/O 124
- Registered Logic 124
- C-14 VHDL Reference Manual 126
- D. ABEL-HDL Language 127
- Dot extensions 128
- VHDL Reference Manual D-7 133
- VHDL Reference Manual D-9 135
- D-10 VHDL Reference Manual 136
- Index-8 VHDL Reference Manual 144
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