Much easier to do in hardware with just a few chips.

555 timer and shift register. A couple more chips and

send R-Y test. You can use a scope or period counter to

accurately setup the 555 clock. Can be implemented in Baudot

or ASCII.

With a uP, use the programmable timer which in most these

days is internal. This should be largely independent of the

code if done right.

Projects like this are fun...enjoy!

Steve W6SSP