Revision 1.02 – June 27, 2006
S5935 – PCI Product
Data Book
Initiator Preemption
Latency Timer register can cause two preemption
situations:
A PCI initiator (bus master) is said to be preempted
when the system platform deasserts the initiator’s bus
grant signal, GNT#, while it still requests the bus
(REQ# asserted). This situation occurs if the initiator’s
latency timer expires and the system platform (bus
arbitrator) has a bus master request from another
device. The S5935 Master Latency Timer register con-
trols the S5935 responsiveness to the removal of a
bus grant (preemption). The presence of a Master
1. Removal of GNT# when the latency timer is non-
zero (S5935 is guaranteed to still “own the bus”).
2. Removal of the GNT# after the latency timer has
expired.
The first situation is depicted in Figure 7, when the
latency timer has not expired. Preemption with a zero
or expired latency timer is shown in Figure 8.
Figure 52. Master Initiated Termination Due to Preemption and Latency Timer Active (S5935 as Master)
6
3
2
4
5
1
PCI CLOCK
GNT #
(I)
(I)
FRAME
IRDY#
TRDY#
(T)
S5933 LATENCY
=3
=1
=0
=2
TIMER
(I) = DRIVEN BY INITIATOR
(T) = DRIVEN BY TARGET
PREEMPTION
DATA
TIMEOUT
SENSED
DATA
DATA
TRANSFERRED
DATA
TRANSFERRED
TRANSFERRED
TRANSFERRED
Figure 53. Master Initiated Termination Due to Preemption and Latency Timer Expired (S5935 as Master)
3
2
4
5
1
PCI CLOCK
GNT #
(I)
(I)
FRAME
IRDY#
TRDY#
(T)
S5933
LATENCY
TIMER
=1
=0
(I) = DRIVEN BY INITIATOR
(T) = DRIVEN BY TARGET
DATA
TRANSFERRED
PREEMPTION
106
DS1527
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