Down Counter can be defined in a simple way don't you think ?
It just counts. HaHa
Yes!! A counter just counts down from the maximum decimal value when all the required bits are 1 to zero.
Let me elaborate.
Let's take an example of 3 bit counter. So whats the maximum of 3bit counter i.e when all bits are 1 in other words "111" It's 7.
Similarly for 4 bits it "1111"that is 15.
Thus n-bits we require n Flip Flops.
Thus an Down counter will count from 15 down to 0 as I have stated and will again start from 15.
Lets code for a 4-bit Down Counter
module up(out,en,clk,reset); output reg[7:0] out; input en, clk, reset; initial out = 15; always @(posedge clk)begin if (reset) begin //if reset is true out <= 8'b00001111 ; end else if (en==1 && out>0) //if enable is high // because 1111 is 15 out <= out - 1; else out <= 15; end endmodule //*************************TestBench module uptest(); reg en; reg clk; reg reset; wire [7:0]out; initial begin reset=0; clk=0; en=1; end up call(out,en,clk,reset); always #2 clk=!clk; endmodule