DDR2 problem with DM6437 for help

Hi,all

     I have got a problem those days. I am using DM6437,but not the EVM board. I can access the DM6437 address through PCI, and I can download program to L2,The program ( led blink test program )runs well in L2,but I can't write or read DDR2.  when I wrote 0x55aa55aa to the 0x80000000, when read I just got 0xfffffff or 0x0000000.

   My DDR2 is NT5TU64M16DG,

    DDR_SDBCR = 0x00138833;

   DDR_SDTIMR = 0
        |(19<<25)   // tRFC  = (  125 ns / 6.17 ns ) - 1
        | ( 1 << 22 )               // tRP  = (  15 ns / 6.17 ns ) - 1
        | ( 1 << 19 )               // tRCD = (  15 ns / 6.17 ns ) - 1
        | ( 1 << 16 )               // tWR  = (  15 ns / 6.17 ns ) - 1
        | ( 6 << 11 )               // tRAS = (  45 ns / 6.17 ns ) - 1
        | ( 8 << 6 )                // tRC  = (  60 ns / 6.17 ns ) - 1
        | ( 1 << 3 )                // tRRD = (  (4*10ns + 2*6.17ns) / 4*6.17ns ) - 1
        | ( 0 << 0 );               // tWTR = ( 7.5 ns / 6.17 ns ) - 1

        DDR_SDTIMR2 = 0

        | ( 12 << 16 )              // tXSNR= ( 85 ns / 6.17 ns ) - 1
        | ( 199 << 8 )              // tXSRD= ( 200 - 1 ) cycles
        | ( 0 << 5 )                // tRTP = ( 7.5 ns / 6.17 ns ) - 1
        | ( 2 << 0 );               // tCKE = ( 3 - 1 ) cycles

     Is there any one who can give me some suggestion  ? 

    I am expecting your reply .

    thank you very much!