SMBus on SiLabs 8051F411,SI stuck low

Please may i have your help?

I can't see high the SI bit(SMBus interrupt flag),even if i set the STA bit,indicating a start condition

The bitrate and timeout timers appears well configured,Pullup resistors are

I use TMR0 as bitrate source and associating its interrupt routine to a narrow pulse i can say the bitrate should be at something less tha 100KHz

Just before to set STA, SMB0CN and SMB0DAT are both zero,SMB0CF is 0xDC.

At this point running the program it never reach the SMBus Interrupt Service Routine(unless i force SI to 1).

Thanks for any help

Diego