Conditional Loop - Sequential Logic Design - Lecture Slides, Slides of Digital Logic Design and Programming

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Sequential Logic Design

Lecture

• Agenda
  1. VHDL : Conditional Loop
  2. VHDL : Attributes
  3. VDHL : Test Benches
• Announcements
  1. n/a

Conditional Loops

• Conditional Loops
  • There are multiple loop structures we can use within VHDL

1. Loop
2. While
3. For

• Loops
  • "Loop" is a keyword that starts a loop
  • creates an infinite loop
  • useful for modeling process that go forever (i.e, clocks, time)

Conditional Loops

• While Loops
  • a Boolean condition is tested at the beginning of the loop
  • the loop only executes if the condition is true

ex) CLOCK_GEN : process begin clock <= '0';

while (EN = '1') clock <= not clock after 1ns; end loop;

end process CLOCK_GEN;

Conditional Loops

• For Loops
  • a loop with a counter
  • the loop executes the # of times in the range that is specified

syntax:

for identifier in range loop

seq-statement seq-statement

end loop;

• the "identifier" is the loop variable.
  • It is implicitly declared when included in the "for" statement.
  • It is automatically the same type as the "range"
  • it will step through ALL values in range

Attributes

• Attributes
  • ability to get more information about a signal other than its current value
  • attributes allow access to the signal's history
    • previous value
    • time since last change
  • this is how we can specify "edge triggered" events in sequential logic
  • we put the attribute keyword after the signal name using the apostrophe (')
  • there are many attributes, the most commonly used are:

1. event
2. transaction
3. last_value
4. last_event

Attributes

• "event" Attribute
  • tells us when there was a change on the signal
  • useful for edge detection

ex) "rising edge"

if (Clock'event and Clock='1')

• "transaction" Attribute
  • tells us when there was an assignment is made to a signal
  • the signal value does not need to change (i.e., 0 to 0)

ex) process (A'transaction)

statement if anybody ever assigns to A