If ce input is high, data bit is stored in the flip/flop and if reset is high flip/flop is cleared. In structural model we have considered that D flip/flop has Control enable input ( ce) along with a reset input. Realization of D flip/flop in structural and behavioral style is below. D flip/flop is used either to store or to delay digital data. module tff(q,reset,clk,t) output reg q input t,reset,clk initial begin q=1'b0 end always (posedge clk) if (reset) q <= 1'b0 else if (t) q= ~q else q = q endmoduleĭ flip/flop is the most used flip/flop in designing digital systems. It is also used in dividing system clock. T flip/flop provides easy logic optimization in designing counters. It is used to generate control signals by changing mode of operation. module jk(q,qb,j,k,reset,clk) output reg q,qb input j,k,clk,reset initial begin q=1'b0 qb=1'b1 end always (posedge clk) if(reset) begin q = 1'b0 qb = 1'b1 end else case(: begin q=~q qb=~qb end endcase endmodule The Verilog code for a JK flip/flop using case statement is shown below. The excitation table can be used to write Verilog code for JK flip/flop. ![]() The drawbacks of SR flip/flop are eliminated in JK flip flop and thus it has no forbidden state. module srff(S,R,clk,reset,q,qb) output reg q,qb input S,R,clk,reset initial begin q=1'b0 qb =1'b1 endĪlways (posedge clk) if (reset) begin q <= 0 qb <= 1 end else begin if (S!=R) begin q <= S qb <= R end else if (S=1 & R=1) begin q <= 1'bZ qb <= 1'bZ end end endmodule So this code is of a NOR gate based SR flip/flop. It can be seen that the state ‘11’ is forbidden. A SR flip/flop is realized using its behavior in behavioral style. This is the most basic flip/flop which is not directly used in any application. The basic four flip/flops are realized below in different coding style. Flip/flops can be realized in any style of Verilog. Here in this tutorial, Verilog implementation of flips/flops will be discussed. ![]() Basics of flip/flops can be found in any basic digital electronic book. A possible positive edge triggering scheme is shown below.įlip/Flops are the basic elements in every sequential circuit. Throughout all the tutorials flip/flops are to be realized as a positive edge triggered. ![]() More on features of the clock will be discussed in the tutorial of static timing analysis. Dual edge triggered flip/flops also exists. Edge triggered flip/flops can be triggered either by a negative edge or positive edge. In reference to the clock signal, a digital logic block can be level sensitive or edge triggered. ![]() A clock signal can be of any duty cycle, any phase and of any frequency. Sequential circuits work with a reference which is the clock. The output of sequential blocks depends on the present state as well as on past state.
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