How to modify OMAPL138 SATA rate

I use the RTFS system to read and write SATA on the OMAP-L138 board. And I use bios_5_41_13_42, rtfs_1_10_02_32, pspdrivers_01_30_01.

I just use the example in ti\rtfs_1_10_02_32\packages\ti\rtfs\examples\sata\bios5\evm6748\ccs4. And I use mini sata.

Now my sata rate is 3Gps, but I want it run at 1.5 Gps. So could the sata rate be modified? And how to relize it?

Best wishes!