ST9 Datasheet, PDF (51/146 Pages) STMicroelectronics – USER GUIDE

ST9 USER GUIDE
ority level as appropriate. The service routine will then allow other interrupts to gain control, at
the expense of delaying its own completion.
Note 1:
In practice, Concurrent mode does not differ much from nested mode if the interrupts are not re-
enabled during an interrupt service routine. Concurrent Interrupts Disabled looks like Nested In-
terrupt Disabled with the difference that the CPL is changed only by software when necessary.
The interrupt requests pending while the interrupt service routine is executing, will only be ser-
viced after the current service routine returns.
Note 2: An interrupt with priority level 7 will never be served.
3.4.3 External Interrupt Unit
This is a functional block that can receive interrupt requests from up to eight external pins, and
also from some internal devices such as the Watchdog Timer, the Serial Peripheral Interface,
etc. It is used to select the active edge for pin or device, and define the priority for each of the
four pairs. In addition, an NMI pin can be programmed as being maskable or non-maskable. It
is maskable on reset, and once set to non-maskable, it cannot again be set to maskable until
the next reset. This works as explained below.
3.4.3.1 Maskable External Interrupt Pins
These are eight external inputs you can set individually to rising-edge or falling-edge sensitive,
and masked. You assign priorities by pairs, making four groups with different priorities. Within
each group, the two interrupt requests have two successive priorities. For example, if you set
group C to priority 2, the INT4 input will have priority 2 and INT5 will have priority 3 (which is
lower).
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