Hi Sascha,

As you can read in the document, the counter is incremented on the falling edge of the input ('1' -> '0'). So for your 24-bit counter you should connect the MSb of your external counter (what type is that, by the way?) directly to the T0 or T1 input. That way the uC counter is incremented when the external counter goes from 'FF' to '00'.

If you invert the MSb, the increment will indeed take place when the external counter reaches '80'.

Best regards,
Rob.