if u want a 8051 solutions . that can be done.
you can read the o/p frequency by keyboard than

1. u can generate the same data into a buffer of 1/3 size of memmory that u need in ur design using the sin series or the f(x) for the shape. rest of method remains the same.

2. u can use external circuitury to convert wave shapes. while sq wave generation is easy (just to flip an i/o line) every 1/2*t

abhishek