Synchronization problem in PWM generation

I paste my code here. It may be helpful.

// PWM Generation.

module PWM_Unit ( duty_ratio,pwm_trig, clk, pwm_output, output_b);

input [15:0] duty_ratio;

input pwm_trig, clk;

output pwm_output, output_b;

integer a=0;

integer b=-1, ref_level=400;

integer pwm_level;

integer half_cycle=0;

reg pwm_output;//output of pwm

reg output_b;//for debug, not related to pwm generation.

parameter steps=500;

always

begin

pwm_output<=pwm_level;

if (b>0)

output_b<=1;

else

output_b<=0;

end

//initialize the pwm unit at the beginning of each switching cycle

always @(posedge pwm_trig)

begin

ref_level=100; // read the duty ratio. this is constant for test and will be change in future.

end

//increase variable a at each clk rising edge

always @(posedge clk)

begin

if (a<ref_level)

pwm_level=1;

else

pwm_level=0;

if (pwm_trig==1) //control the slope of the triangle wave.

b=-1;

if (pwm_trig==1)

b=1;

a=a+b;

if (a<1)

begin

a=0;

end

if (a>(steps-1))

begin

a=steps;

end

end

endmodule