module johnson #(parameter N=8)
(output logic [N-1:0] q, input logic clk,reset);
always_ff @(posedge clk,posedge reset)
if(reset)
q<=0;
else
q<={~q[0],q[N-1:1]};
endmodule
Above is the systemverilog HDL for an 8-bit Johnson counter. I read from a textbook that it has large number of unused states that form a counter of their own i.e a parasitic state machine. What exactly is this parasitic state machine?
The problem here is that if this circuit ended up entering one of the unused states for some reason you would be stuck in a loop then never returns to one of the used states. This counter has the following states:
00000000
10000000
11000000
11100000
11110000
...
00001111
00000111
00000011
00000001
If a cosmic ray hits your chip and flips one of the bits then you could end up with states that don't exist in normal operation:
00010000
10001000
11000100
11100010
11110001
01111000
10111100
....
The only way to get back to normal is by asserting reset. You have two options to deal with this: either add a bunch of logic to detect the situation, or assume that a bit randomly flipping isn't going to happen. I would say that you don't need to worry about it unless that flipped bit is going to launch a missile.
