PNX1311EH NXP Semiconductors, PNX1311EH Datasheet - Page 276

• Manufacturer Part Number: PNX1311EH
• Manufacturer: NXP Semiconductors

Specifications of PNX1311EH

• Lead Free Status / RoHS Status: Not Compliant

PNX1300/01/02/11 Data Book
ARB_RAISE register contents has been changed for the
application requirements.
Corner-case note: There is some risk in setting the delay
high, then lowering it, as the last request submitted with
the high delay might violate the latency constraints of the
new real-time domain. However this should not happen
since this register should be set before the application
starts.
The other units (AI, AO and BTI (boot block)) and the
CPU will always have their requests considered as high
priority. High priority for the CPU will give maximum pos-
sible performance.
AO and AI requests are happening at very low rate.
Hence, the probability that they take time away from the
CPU is negligible.
20.3
In addition to the dual priority mechanism, a round-robin
arbitration is used to schedule the requests with same
priority. The purpose is to ensure, for every unit with a
high-priority request, a maximum latency for gaining ac-
cess to the highway and/or a minimum share of the avail-
able bandwidth.
Round-robin arbitration ensures that no starvation of re-
quests can occur and therefore requests with real-time
constraints can be handled in time.
The round robin arbitration algorithm is as follows.
Requests are granted according to a dynamic priority list.
Whenever a unit request is granted, it will be moved to
the last position in the priority list and another unit will be
moved to the first position in the priority list. Priorities are
rotated. A unit with a waiting request will eventually reach
the first place in the priority list.
As an example,
arbitration state machine with 2 requesters. The nodes A
and B indicate states A and B. In state A, requester A has
ownership of the highway, in state B requester B has
ownership. The arc from state A to state B indicates that
if the current state is state A and a request from request-
er B is asserted, then a transition to state B occurs, i.e.
ownership of the highway passes from requester A to re-
quester B.
When, in a particular state, none of the arcs leaving from
that node has its condition fulfilled, the state machine re-
mains in the same state.
20-2
ROUND ROBIN ARBITRATION
Figure 20-1
PRELIMINARY SPECIFICATION
shows a state diagram of an
When both requester A and B have requests asserted,
then ownership of the highway switches between A and
B, creating fair allocation of ownership.
Figure 20-2
arbitration with 3 requesters.
20.3.1
Not all units need to have equal latency and bandwidth.
It is preferred to allocate bandwidth to units according to
their needs. This is achieved with weighted round-robin
and can be illustrated in the following examples.
Figure 20-3
A and B with double weight given to requester A. There
are now 2 states A1 and A2 where requester A has own-
ership of the highway. When both A and B requests are
asserted, requester A will have ownership of the highway
twice as often as requester B.
Figure 20-3. State diagram of round robin arbitra-
tor with 2 requesters; A has double weight.
Figure 20-1. State diagram of round robin arbitra-
tor with 2 requesters.
Figure 20-2. State diagram of round robin arbitra-
tor with 3 requesters.
Weighted Round Robin Arbitration
A
A
A
C&~B
A1
A2
pictures a state machine with two requesters
pictures a state diagram that allocates fair
A
B
A
Philips Semiconductors
B&~A
A
A&~C
B
C
B
C
B&~A
B
B
B