This is the first component in a series of five exercises that builds a complex counter out of several smaller circuits. See the final exercise for the overall design.
Build a four-bit shift register that also acts as a down counter. Data is shifted in most-significant-bit first when shift_ena is 1. The number currently in the shift register is decremented when count_ena is 1. Since the full system doesn't ever use shift_ena and count_ena together, it does not matter what your circuit does if both control inputs are 1 (This mainly means that it doesn't matter which case gets higher priority).
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module top_module (
input clk,
input shift_ena,
input count_ena,
input data,
output [3:0] q);
reg [3:0] q_temp;
always @(posedge clk) begin
if(shift_ena) begin
q_temp <= {q_temp[2:0],data};
end
else if(count_ena) begin
if(q_temp == 4'd0) begin
q_temp <= 4'd15;
end
else begin
q_temp <= q_temp - 1'b1;
end
end
end
assign q = q_temp;
endmodule
