Hi Peter,
Thanks for your help.
I have found something wrong and the SWT issue has solved.
In my project startup.s file, the bit of MAP0 need to enable so that 0x7F00 modified into 0xFF00.
Show as the following code:
e_lis r4, 0xFC05
e_or2i r4, 0x0000
e_li r3, 0xC520
e_stw r3, 0x10(r4)
e_li r3, 0xD928
e_stw r3, 0x10(r4)
//e_lis r3, 0x7F00 fail
e_lis r3, 0xFF00 //good
e_or2i r3, 0x010A
e_stw r3, 0(r4)
Now the SWT works well and could enter SWT interrupt after second overflow time.