Sincerely for this peripheral i advise you reading the datasheet chapter "15 Synchronous Serial Interface (SSI)"
I was going to write an explanation about this peripheral but the datasheet explain really well not only about the SSI but also the supported frame formats (protocols): Freescale SPI, MICROWIRE and Texas Instruments synchronous serial.
Of course I will put up some examples in the future and more info that I find pertinent.