STR911 ADC analog watchdog

I want to use ADC in AWD-Mode. This means the ADC shall generate an 
interrupt when a threshold at port 4.0 is exceeded. But I do not get it 
to run.
If I read ADC_DR0 correct data are read from analog input channel 0.
In ADC_CR I set the bits 9(AWDI), 4(CONT), 1(POR), 0(STR).
I set ADC_CCR to 0x0002 and ADC_HTR to the required threshold.
The ADC_CRR register does not change when the threshold is exceeded and 
the AWD-bit in ADC_CR is not set.
Does anyone has an idea whats wrong and does anybody got the AWD to run?

Thanks in advance
Pedro