C515A_9708 Datasheet, PDF (89/182 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

On-Chip Peripheral Components
C515A
The second configuration which should be noted is when compare function is combined with
negative transition activated interrupts. lf the port latch of port P1.0 contains a 1, the interrupt
request flags IEX3 will immediately be set after enabling the compare mode for the CRC 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 lf the
request flag is cleared by software after the compare is activated and before the external interrupt
is enabled.
Semiconductor Group
6-38
1997-08-01