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ASYNCHRONOUS COUNTER

Asynchronous means in terms of simple definition without external
clock synchronization.

The output always remains free from clock signal.

Generally the first FF is clocked with main
external clock and each of next FF have output of previous FF as their clock.
This helps in reducing the number of FFs and additional gates hence requires
less complexity.

Now coming to the special "MOD" term. It
basically stands for modulus.

When you have to design a Mod-Y counter then the basic steps
include

1. The equation -: 2

^{x }= Y.
2. Now find value of X if you know basic
Maths. You can use logarithms

Thus after getting the value of X you
basically get how many FFs are required hence you require X FFs to design Mod-
Y UP or Down counter.

Here is the block diagram of Mod-16 or
4bit Asynchronous Counter

Now
For Mod-16 we have value of X as 4 hence 4 FFs

Here is the code to test this

*module dff(d,clk,q);*

*input d;*

*input clk;*

*wire d;*

*wire clk;*

*output q;*

*reg q;*

*initial*

*q = 1'b0;*

*always @(posedge clk)begin*

*q <= d;*

*end*

*endmodule*

*//************************Test Bench******************//*

*module FOURbit_up_B();*

*reg clk;*

*reg d;*

*wire [3:0]q;*

*initial*

*clk = 0;*

*always*

*#1 clk = !clk;*

*dff a1(!q[0],clk,q[0]);*

*dff a2(!q[1],!q[0],q[1]);*

*dff a3(!q[2],!q[1],q[2]);*

*dff a4(!q[3],!q[2],q[3]);*

*endmodule*

This
was Aynchronous Up counter.

Pay attention here that clock input to
each FF is

**~Q (Q bar)**. Thus for Down counter the clock input after 1st FF will be from**Q and not ~Q (Q bar)**
Here is the block diagram for 4 bit
Down Asynchronous Counter

Notice
the clock inputs to each FF after 1st FF.

Here is the code for Down Counter 4 bit

*module dff(d,clk,q);*

*input d;*

*input clk;*

*wire d;*

*wire clk;*

*output q;*

*reg q;*

*initial*

*q = 1'b0;*

*always @(posedge clk)begin*

*q <= d;*

*end*

*endmodule*

*module FOURbit_up_B();*

*reg clk;*

*reg d;*

*wire [3:0]q;*

*initial*

*clk = 0;*

*always*

*#1 clk = !clk;*

*dff a1(!q[0],clk,q[0]);*

*dff a2(!q[1],q[0],q[1]);*

*dff a3(!q[2],q[1],q[2]);*

*dff a4(!q[3],q[2],q[3]);*

*endmodule*

and
heres the simulation wave window from Xilinx iSim

Mail or Comment for any
flaws here or if you have any doubts.

So Long

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