在u-boot烧写u-boot.ais到nand flash，无法从nand flash启动的原因？

用创龙的OMAPL138-EVM开发板，想在u-boot烧写u-boot.ais到nand flash，无法从nand flash启动。用创龙提供的nand-write.out通过ccs烧写u-boot.ais后就能启动。但是查看hawkboard写着可以通过u-boot烧写u-boot.ais到nand flash，请问要怎么修改才能在u-boot烧写u-boot.ais到nand flash，然后能从nand flash启动？
hawkboard 在网上的烧写步骤是：Procedure to flash u-boot on NAND

After booting the u-boot over UART as mentioned above,

• On the u-boot prompt in the terminal window
• Configure the Ethernet server and Client IP addresses, For e.g.
  

  hawkboard.org > setenv serverip 172.24.156.199  hawkboard.org > setenv ipaddr 172.24.190.58

• Download the u-boot generated for NAND
  

  hawkboard.org > tftpboot 0xc0700000 u-boot_nand_ais.bin    TFTP from server 172.24.156.199; our IP address is 172.24.190.58  Filename 'u-boot_nand_ais.bin'.  Load address: 0xc0700000  Loading: #################################################################           #################  done  Bytes transferred = 414988 (6550c hex)  

• Erase NAND Flash
  

  hawkboard.org > nand erase  NAND erase: device 0 whole chip  OK

• Flash the NAND with u-boot
  

  hawkboard.org > nand write.e 0xc0700000 0x20000 0x70000  NAND write: device 0 offset 0x20000, size 0x70000  458752 bytes written: OK  hawkboard.org >

• Switch off the board
• Change the DIP Switches for NAND boot 1-ON 2-OFF 3-OFF 4-OFF