Consider:

  #define T1 20/3

Evaluated on its own that will be truncated to 6,
whereas 20.0/3.0 will give the correct value.

But also consider:

  2*PI*t/T1

Using the above defined value for T1 that becomes:

  2*PI*t/20/3 = (2*PI*t/20)/3 = (2*PI*t)/(20*3)

which is not correct.

The correct way to define T1 is:

  #define T1 (20.0/3.0)