C509-L_97 Datasheet, PDF (150/290 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

On-Chip Peripheral Components
C509-L
Please note that for CC registers 1 to 3 an interrupt is always requested when the compare signal
goes active.
The second configuration which should be noted is when compare functions are combined with
negative transition activated interrupts. lf the port latch of port P1.0 or P.1.4 contains a 1, the
interrupt request flags IEX3 or IEX2 will immediately be set after enabling the compare mode for the
CRC or CC4 register. The reason is that first the external interrupt input is controlled by the pin’s
level. When the compare option is enabled the interrupt logic input is switched to the internal
compare signal, which carries a low level when no true comparison is detected. So the interrupt
logic sees a 1-to-0 edge and sets the interrupt request flag.
An unintentional generation of an interrupt during compare initialization can be prevented if the
request flag is cleared by software after the compare is activated and before the external interrupt
is enabled.
Semiconductor Group
6-72
1997-10-01