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Lec22 - VERILOG HDL DESIGN Lec 22 Timing delays module...

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VERILOG HDL DESIGN Lec 22
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© BITS, Pilani Timing delays module code11 (out1, out2, in); output out1, out2; input in; reg out1, out2; always @(in) begin #25 out1 = ~in; #40 out2 = ~in; end endmodule An always block that does not schedule events in zero time could miss RTL- or behavioral model triggered events. First, once the always block is entered due to a change on the sensitivity list variable in , subsequent changes on in will not cause re-entry until the always block is exited 65 time units later. Second, after a delay of 25 time units, the current value of in is read, inverted, and assigned to out1 . After an additional 40 time units, in will again be read, inverted, and assigned to out2 . During the timing delays, all other events on in will be ignored. The outputs will not be updated on every input change if changes happen more frequently than every 65 time units. The post-synthesis gate-level model will simulate two inverters while the pre- synthesis RTL code will miss multiple input transitions Placing delays on the left side of always block assignments does not accurately model either RTL or behavioral models.
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© BITS, Pilani Blocking and Nonblocking assignments A Verilog race condition occurs when two or more statements that are scheduled  to execute in the same simulation time step, would give different results when the  order of statement execution is changed, as permitted by the Verilog Standard Execution of blocking assignments is a one-step process: •Evaluate the RHS (right-hand side argument) and update the LHS (lefthand side argument) of the  blocking assignment without interruption from any other Verilog statement •"Blocks"  trailing  assignments  in  the  same  always  block  from  occurring  until  after  the  current  assignment  has  completed
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