Linux/DS90UB925Q-Q1: Internal Pattern Generation with external video timing

Part Number:DS90UB925Q-Q1

Tool/software: Linux

I have tested the Internal Pattern Generation with  internal video timing on UB925.This has been successful

Now ,I want to test the   Internal Pattern Generation with  external video timing on UB925.I have  entered HS, VS, DE, PCLK signals on the 925 pin.

But,This time I can get nothing on my LCD.

I have config UB925 like this:

0x03=0xab

0x04=0x8a

0x12=0x4a

0x65=0x01

0x64=0x01

Should I config some othe registers ?

regards,

chengbo