For what application?

Hi,

Asynchronous Bit-Serial Intefacing(Data transmission).

Double buffering allows you to read or write data to the parallel access side while data is being shifted in or out of the shift register. Without it, you would have to read or write syncronous with the shift clock.

Thanks to you both:

But I'm still unclear on the whole picture.
I mean, is this used in all asynchronous data transmission systems?

Sorry about my lack of understanding.

No, but it makes things a lot easier when it is.

You can design a serial interface chip without it, but it means that the program has to monitor the transmit buffer empty flag almost continuously, not leaving much time for any other processing.

Double buffering enables the transmit buffer to be reloaded automatically reducing the load on the micro by transferring data from the transmit input buffer to the transmit buffer when the transmit buffer empties (last bit shifted out).

This gives the micro a whole character transmission period to reload the input buffer, which is a lot less critical.

The technique is used in other applications (like A2D and D2A) but the serial case is easiest to understand.

Thanks Zeitghost, this makes me a lot clearer.