Linux/AM5718: No serial debug output

Part Number: AM5718

Tool/software: Linux

Hi TI Experts,

I am a fresh developer for Sitara Processors, I meet one problems for booting the uboot from SD card. I create the SD card by 

http://processors.wiki.ti.com/index.php/Processor_SDK_Linux_Creating_a_SD_Card_with_Windows

The image is download from 

http://software-dl.ti.com/processor-sdk-linux-rt/esd/AM57X/latest/index_FDS.html

 According to office doc sprui97.pdf, 4.1 The  AM571x IDK EVM is configured by default to 0x8106 to enable UBOOT/Linux boot from the SDCARD. 

I using minicom to connect the serial port from the AM571x IDK EVM, the setting is : baud: 115200 data bits: 8 parity: N Stop Bits: 1.

But I can not seen any log from the minicom terminal. Any problems for my process?

Thanks a lot!

Nemo