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Altera_Forum
Honored Contributor I
1,208 Views

flop-flop simulation in ModelSim

i try to simulate [1:0]flip-flop in ModelSim and i see one normal signal(out_inf[0]) and one blue signal(out_inf[1]). What is it?

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6 Replies
Altera_Forum
Honored Contributor I
72 Views

Blue means 1'bZ. We will need to code to understand the problem.

Altera_Forum
Honored Contributor I
72 Views

dff code 

module dff(clk, din, dout); input clk; input din; output dout; reg dout; always @ (posedge clk) begin dout <= din; end endmodule  

 

testbench 

module top; reg clk; reg in_inf; wire out_inf; dff D1 (clk, in_inf, out_inf); initial // Clock generator begin clk = 0; forever# 10 clk = !clk; end initial //in_inf begin in_inf = 0; # 28 in_inf = 1; # 5 in_inf = 0; end initial //in_inf begin in_inf = 0; # 48 in_inf = 1; # 5 in_inf = 0; end endmodule  

i want to see normal (not blue) signal as out_inf[1]
Altera_Forum
Honored Contributor I
72 Views

You should always look sharp at the Modelsim warnings. They tell about problem with reg dout declaration. Must be reg [1:0] dout. Or include reg in the output definition.

Altera_Forum
Honored Contributor I
72 Views

whys out_inf[0] is h'x (in the beginning), not zero?

Altera_Forum
Honored Contributor I
72 Views

Because you dont have an initial or reset value for them. They wont be assigned a value until the first clock

Altera_Forum
Honored Contributor I
72 Views

can i initialize value before first clock?

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