Here’s an example Verilog code to implement a 4-bit Johnson counter circuit:
module johnson_counter (output reg [3:0] count, input clk);
always @(posedge clk) begin
count <= {count[2], count[3],
count[2],
count[3]};
end
endmodule
This code defines a module called "johnson_counter" that implements a 4-bit Johnson counter. The output "count" is a registered output that represents the current state of the counter. The input "clk" is the clock input that controls the counter operation.
The "always @(posedge clk)" block is a sequential logic block that updates the "count" output based on the clock signal. On every positive edge of the clock, the counter shifts the current contents of "count" left by two bits and inserts the complement of the second and third bits at the most significant bit positions, and the second and third bits at the least significant bit positions. This creates a sequence of 8 states in which each state differs from its adjacent states by only one bit.
Note that this Verilog code assumes that the "count" output and the "clk" input are both 1-bit signals. If you want to use multi-bit signals, you would need to modify the code accordingly. Additionally, this code implements a rising-edge triggered counter. If you wanted to implement a falling-edge triggered counter, you would need to modify the code accordingly.