Can't boot by u-boot2

Hello all,
I am trying to do serial/usb boot via u-boot2.
After downloading by pserial/omap3_usbload, the terminal output is a
lot of "�����������������"
I can't see the correct message but the console actually response, I
can type something with keyboard and the number of "�" grows.

The result remain the same even I build a new u-boot.bin from source
download at the u-boot 2 git(git clone git://

How do I fix the problem that display messages shows incorrect?

I am using the Beagleboard xM and the host is ubuntu 9.04 .

Thanks very much.