Motorola DSP56156 Manual page 225

SSI OPERATING MODES
will not generate a periodic frame sync. A frame sync pulse will be generated only when
there is data available to transmit. The On-demand mode requires that the transmit frame
sync be internal (output) — i.e., the on-demand mode is disallowed whenever the transmit
frame sync is configured as an input. Data transmission is data driven and is enabled by
writing data into the TX register. Receive and transmit interrupts function with the TDE and
RDF flags as normally; however, transmit underruns (TUE) are impossible for on-demand
transmission.
8 - 28 SYNCHRONOUS SERIAL INTERFACE (SSI0 and SSI1) MOTOROLA