ADCBUSY pin on ADuC7024

On the ADuC7024 I have tried using P0.5 to output the ADCBUSY signal, but it doesn't seem to interact correctly with bit 6 (Enable ADCBUSY) of ADCCON.

I set GP0CON to 0x00100000 to map ADCBUSY to P0.5 then set ADCCON = 0x00000620 to power up the ADC and then configure ADCCON = 0x000017A4.

At this point a continuous pulse train starts on P0.5 with a +width of 25usec.

I would expect that clearing bit 6 of ADCCON would inhibit the output of ADCBUSY on P0.5.

Can you suggest what might be causing this?