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synchronous serial bidirectional master mode

Hello all,

I have a question about the synchrounous serial ports of the
etrax 100LX.
We're thinking about using a synchronous serial port in 'master
bidirectional mode', but it seems the examples in the documentation
only describe single direction communication.

Maybe someone can point me to a good datasource with descriptions
of the various synchronous serial modes.

It looks like there is no indication for the slave to tell the
master it has data to transmit. So the master probably has to
keep sending dummy data to generate frame pulses, in order to
allow the slave to send data back?
Or are the frame pulses continuously generated, even when the master
has no data to transmit?

And when the slave has no data to transmit, the master will probably
receive frames filled with invalid data, for each frame pulse it generates?
Or is there a way in the synchronous serial protocol for the slave 
to indicate it has data to send, so that the DMA buffers of the
master only get filled with valid data?

As you see, I'm new to the concept of synchronous serial communication,
anything that can point me into the right direction would be greatly