// music_ambulance_siren.v   //救護(hù)車汽笛
/**********************************************************************
Let's alternate 2 tones. We first use a 24 bits counter "tone" to produce 
a slow square wave. The MSB bit (tone[23]) toggles with a frequency of about 1.5Hz.
Then we use this bit to switch between 2 frequencies for the main counter,
 and that alternates between the 2 tones. Here comes the ambulance!

**********************************************************************/

module music_ambulance_siren(clk, speaker);
input clk;
output speaker;
parameter clkdivider = 25000000/440/2;

reg [23:0] tone;
always @(posedge clk) tone <= tone+1;

reg [14:0] counter;
always @(posedge clk) 

if(counter==0) counter <= (tone[23] ? clkdivider-1 : clkdivider/2-1);

 else counter <= counter-1;

reg speaker;
always @(posedge clk) if(counter==0) speaker <= ~speaker;

endmodule