Hi Jim,
You could read more about SSI in the Stellaris data sheet chapter 15: http://www.ti.com/lit/ds/symlink/lm4f232h5qd.pdf
The Stellaris® microcontroller includes four Synchronous Serial Interface (SSI) modules. Each SSI
module is a master or slave interface for synchronous serial communication with peripheral devices
that have either Freescale SPI, MICROWIRE, or Texas Instruments synchronous serial interfaces.
Pedro