24 June 2012

Verilog HDL: Timing and Delays In Verilog

Timing  and  Delays In Verilog:
Path  Delay  Modeling :-
Specify  Blocks :
->a  delay  between  a source  (input  or  inout)  pin  and  a  destination (output  or  inout)  pin   of  a  module  is  called  a   module path delay
->keywords for path delay : ‘specify’,’endspecify’

Eg :
module  m(out,a,b,c,d) ;
output  out;
input  a,b,c,d;
wire  e,f;
specify       //specify  block with path  delay  statements
         (a => out) = 9;  //for  full connection : (a,b*>out) = 9;
         (b=> out) = 9; 
         (c => out) = 11;  //for  full connection : (c,d*>out) = 11;

         (d => out) = 11; 
endspecify
and a1(e,a,b);
and a2(f,c,d);
and a3(out,e,f);
endmodule

Note :
Specify block  is  a  separate  block  in  the  module  and  does  not  appear  under  any  other  block, such  as  initial  or  always.
“specparam”  à  special  parameter  declaration  for  use inside  a specify  block ;  same  as  ‘parameter’   but  only  within  ‘specify’  block.

Rise, fall  and turn off delays :-
->if  only  one  delay  is  specified  , this  value  is  used  for  all  transitions.
->if  two  delays  specified, they  refer  to  the  rise  and fall delay values. The turn-off  delay
Is the minimum of  the  two delays.
->if  all  3  delays  are  specified, they refer  to rise, fall and turn-off delay values.
Eg :   and   #(5)  a1 (out,i1,i2);   //delay  of   5 for all transitions
          and   #(4,6)  a2 (out,i1,i2);  //rise=4,fall=6
          bufif0  #(3,4,5)  b1(out,in,control);   //rise=3,fall=4,turn-off=5


Min/Typ/Max   Values :
Eg1 :  and  #(4:5:6)   a1(out,i1,i2);
Here  4  is minimum  delay(+min delays)
5 is typical  delay(+ty  delays)
6  is  maximum delay(+max  delays)

Eg2 :
and   #(2:3:4,3:4:5,4:5:6)  a3(out,i1,i2);
           Here  2-rise,3-fall,4-turn off  min delays
           3-rise,4-fall,5-turn off  typdelays
            4-rise,5-fall,6-turn off  maxdelays


Timing  checks : $setup  and   $hold :
->$setup,  $hold,  $width
->all  timing  checks  must  be  inside  ‘specify’  blocks
$setup  task  :
$setup (data_event,reference_event,limit);
Data_event : signal that is monitored for violations
Reference_event :  signal that establishes  a  reference  for  monitoring  the  data_event  signal
Limit  :minimum  time  required  for  setup  of  data  event.

$hold   task :
$hold  (reference_event,data_event,limit);

$width  task :
$width (reference_event,limit);
Reference_event : Edge  triggered  event(edge  transition  of  a  signal)
Limit  :  minimum  width  of  the  pulse

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