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TC1796
System Units (Vol. 1 of 2)
Interrupt System
User’s Manual
14-7
V2.0, 2007-07
Interrupt, V2.0
14.2.1.6 Service Request Priority Number (SRPN)
The 8-bit Service Request Priority Number (SRPN) indicates the priority of a service
request with respect to other sources requesting service from the same service provider,
and with respect to the priority of the service provider itself.
Each active source selecting the same service provider must have a unique SRPN value
to differentiate its priority. The special SRPN value of 00
H
excludes an SRN from taking
part in arbitration, regardless of the state of its SRE bit. The SRPN values for active
sources selecting different service providers (CPU vs. PCP) may overlap. If a source is
not active – meaning its SRE bit is 0 – no restrictions are applied to the service request
priority number.
The SRPN is used by service providers to select an Interrupt Service Routine (ISR)) or
Channel Program (in case of the PCP) to service the request. ISRs are associated with
Service Request Priority Numbers by an Interrupt Vector Table located in each service
provider. This means that the TC1796 Interrupt Vector Table is ordered by priority
number. This is unlike traditional interrupt architectures in which their interrupt vector
tables are ordered by the source of the interrupt. The TC1796 Interrupt Vector Table
allows a single peripheral can have multiple priorities for different purposes.
The range of values for SRPNs used in a system depends on the number of possible
active service requests and the user-definable organization of the Interrupt Vector Table.
The 8-bit SRPNs permit up to 255 sources to be active at one time (remembering that
the special SRPN value of 00
H
excludes an SRN from taking part in arbitration).
Note: Before modifying the content of a SRPN bit field, the corresponding SRN must be
disabled (SRE = 0).
SRPNs in the TC1796
In the TC1796, interrupt sources selecting the same service provider are also allowed to
have identical SRPN values. In this case, the software (interrupt service routine) must
check which of the interrupt sources with identical SRPN has become active.
Note that module-specific interrupt request flags must be available because the SRR
flags cannot be used for this check. SRR flags (meaning all SRR flags of interrupts with
identical SRPN values) are in general automatically cleared by hardware when a service
request is acknowledged and serviced.
Note: This practice with identical SRPN values is not recommended as it is not portable
to other TriCore devices.