If you fabricate an interrupt service routine that responds to the 2400 Hz clocking on the external interrupt routine it should be possible to make it so that the some bit is sent to the P0.x pin at each interrupt. Just keep in mind that at 2400 Hz the time period from interrupt to interrupt is about 417 µsec. Feasibility will depend upon a number of factors including:

1) How many instructions your processor can execute in 417 µsec.

2) Whether or not the 2400 Hz interrupt is the only interrupt that is being used.

3) Whether or not your choice of processor lets you pick an oscillator rate that provides suitable performance.

4) Whether or not you are stuck with a OSC/12 instruction rate or there are choices to use one of the newer OSC/6, OSC/4, OSC/2 or OSC/1 type parts.

You may also want to consider the use of the UART in the shift register mode with external clocking. Now the processor only need load the register once each byte time as opposed to sending a bit every bit cell time.

Michael Karas