Representing Edges Using Signal Attributes in VHDL

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Representing Edges UsingSignal Attributes in VHDL
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• Attributes

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Attributes (I/II)(pp. 75-76, Yalamanchili, "VHDL
Starter's Guide")

• signal_name'event
• returns true if there is a change in a value
  of the signal.
• signal_name'active
• returns true if a value was assigned to the
  signal (even if the new value is the same as the
  current signal value).
• signal_name'last_event
• returns the elapsed time since the last
  event that happened to the
• signal.
• signal_name'last_active
• returns the elapsed time since the last time
  that the signal was
• active.
• signal_name'last_value
• returns the last value that was assigned to the
  signal.

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Attributes (II/II)

• scalar_name'left
• returns the leftmost value in the definition
  of scalar_names data type.
• scalar_name'right
• returns the rightmost possible value (in the
  data type)
• scalar_name'high
• returns the highest possible value
• scalar_name'low
• returns the lowest possible value
• scalar_name'ascending
• returns true if the values in the definition
  are in increasing order.
• array_name'length
• returns the number of elements in the vector

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• Attribute Examples in Detecting Clock Edges

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Representing Edges in VHDL

• A clock change is represented using
• CLK' event
• Which returns 'true after the change to the
  clock.
• For a positive edge
• CLK' event and CLK'1'
• Which returns true after the change that led
  the clock to the value 1
• (from a value of 0).
• For a negative edge
• CLK' event and CLK'0'

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Edge Detection in the std_logic_1164 Package
-------------------------------------------------
------------------ -- edge detection
--------------------------------------------------
----------------- FUNCTION rising_edge
(SIGNAL s std_ulogic) RETURN BOOLEAN IS
BEGIN RETURN (s'EVENT AND (To_X01(s)
'1') AND
(To_X01(s'LAST_VALUE) '0')) END
FUNCTION falling_edge (SIGNAL s std_ulogic)
RETURN BOOLEAN IS BEGIN RETURN
(s'EVENT AND (To_X01(s) '0') AND
(To_X01(s'LAST_VALUE) '1'))
END The function TO_XO1(.) converts inputs to
'?', '0', or '1'. For detecting clock edges,
we must use the functions given above!