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The BCD (Binary Coded Decimal) counter is simply a counter that resets when the decimal value 10 is reached instead of the normal 15 for a 4-bit binary counter. This counter is often used for decimal displays and other human interface hardware. The VHDL for a BCD counter is very similar to that of a basic binary counter except that the maximum value is 10 (hexadecimal A) instead of 15 (hexadecimal F). The VHDL for a simple BCD counter is given in the following listing. The only change is that the counter has an extra check to reset when the value of the count variable is greater than 9 (the counter range is 0 to 9).
1 library ieee;
2 use ieee.std_logic_1164.all;
3 use ieee.numeric_std.all;
4
5 entity counter is
6 generic (
7 n : integer := 4;
8 port (
9 clk : in std_logic;
10 rst : in std_logic;
11 output : out std_logic_vector((n−1) downto 0)
12 );
13 end;
14
15 architecture simple of counter is
16 begin
17 process(clk, rst)
18 variable count : unsigned((n−1) downto 0);
19 begin
20 if rst = ’0’ then
21 count := (others => ’0’);
22 elsif rising_edge(clk) then
23 count := count + 1;
24 if count > 9 then
25 count := 0;
26 else if
27 end if;
28 output <= std_logic_vector(count);
29 end process;
30 end;
In a similar manner we can implement a BCD counter in Verilog using the code given here:
1 module bcd_counter (
2 clk, / clock input
3 rst, / reset (active low)
4 counter_output / counter output
5 );
6
7 input clk;
8 input rst;
9
10 output [3:0] counter_output;
11
12 wire clk;
13 wire rst;
14
15 reg [3:0] counter_output ;
16
17 always @ (posedge clk)
18 begin : count
19 if (rst == 1’b0) begin
20 counter_output <= #1 4’b0000;
21 end
22 else begin
23 if(counter_output < 9) begin
24 counter_output <= #1 counter_output + 1;
25 end
26 else
27 counter_output <= #1 4’b0000;
28 end
29 end
30
31 endmodule
Figure 24.7 BCD counter simulation.
and test it using the same basic counter test bench created for the simple counter, giving the simulation results as shown in Figure 24.7. In the results this time you can see the counter variable in binary and also in unsigned decimal counting up to 1001 (binary) and 9 (decimal), then returning back to 0, giving the decimal counter.
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