MPC563XM Reference Manual, Rev. 1
792
Freescale Semiconductor
Preliminary—Subject to Change Without Notice
Figure 23-36. Priority Passing Disabling Example
23.4.3.2.3
Secondary Scheme - Priority Among Channels on the Same Level
Because channels can randomly request service, channels having the same priority level will inevitably
request service simultaneously. A secondary scheme prioritizes these requests. The Scheduler services
channels on each of the three priority levels, beginning with the lowest numbered channel on that level.
23.4.3.2.4
Priority Scheme Example
The overall priority scheme simultaneously incorporates both primary and secondary schemes. Combining
both schemes in the following example conveys their correlation.
1. One high-priority and one low priority channels request service, while the Scheduler is in time slot
one. Having its service request bit asserted, a single high-level channel is granted the time slot,
which has high-level priority (primary scheme) and its service grant bit is asserted. At the end of
the thread, the service grant bit is negated (no more requests of high priority level channels).
2. The Scheduler proceeds to time slot two, which has middle-level priority; however, no
middle-level channel is requesting service. Priority is passed to the high level, but no high-level
channel is requesting service; therefore, priority is passed again, and service is granted to the single
requesting low-level channel. Once serviced, this channel’s grant bit is negated (no more low-level
requests).
3. The Scheduler resumes with the fixed-priority sequence on time slot three; however, no channels
are requesting service. The Scheduler returns to time slot one, waiting for requests.
SLOT Number
6
7
1
2
4
4
5
7
4
6
4
1
2
M
H
H
M
L
L
H
H
M
L
M
L
H
M
High Pend Count
Service High
2
1
0
1
0
2
0
- X New Service Requests Arrive at a Specific Priority Level
2
0
X
2
2
2
1
1
1
0
0
0
1
2
2
1
0
2
2
2
DH
DM
DH
DL
DL
DH
DH
DM
DM
DL
DM
DL
1
ID
0
DH, DH, DL - Default Service High, Middle or Low
ID - Idle (no service request)
ID
Slot Assignment
SLOT ASSIGNMENTS:
Fixed Priority Level
Middle Pend Count
Service Middle
Low Pend Count
Service Low
Cycle A
Cycle B
Cycle C (truncated)
Cycle D
3
5
H
M
1
1
1
1
0
1
1
0
0
1
H
DM
DH
DM
Reset Slot
Number