Revision 1.02 – June 27, 2006
S5935 – PCI Product
Data Book
Bus Acquisition
potentially monopolizing the PCI bus and also allows
more accurate estimations for bus access latency.
Once GNT# is asserted, giving bus ownership to the
S5935, the S5935 must wait until the PCI bus
becomes idle. This delay is called bus acquisition
latency and involves the state of the signals FRAME#
and IRDY#. The current bus master must complete its
current transaction before the S5935 may drive the
bus. Table 3 depicts the four possible combinations of
FRAME# and IRDY# with their interpretation.
Target Locking
It is possible for a PCI bus master to obtain exclusive
access to a target (“locking”) through use of the PCI
bus signal LOCK#. LOCK# is different from the other
PCI bus signals because its ownership may belong to
any bus master, even if it does not currently have own-
ership of the PCI bus. The ownership of LOCK#, if not
already claimed by another master, may be achieved
by the current PCI bus master on the clock period fol-
lowing the initial assertion of FRAME#. Figure 15
describes the signal relationship for establishing a
lock. The ownership of LOCK#, once established, per-
sists even while other bus masters control the bus.
Ownership can only be relinquished by the master
which originally established the lock.
Target Latency
The PCI specification requires that a selected target
relinquish the bus should an access to that target
require more than eight PCI clock periods (16 clocks
for the first data phase in a burst). Slow targets can
exist within the PCI specification by using the target
initiated retry. This prevents slow target devices from
Figure 60. Engaging the LOCK# Signal
3
6
2
45
1
PCI CLOCK
(I)
FRAME #
LOCK #
AD[31:0]
IRDY#
STILL DRIVEN BY PREVIOUS
OWNER (TARGET IS LOCKED)
(T)
(I)
(I)
DATA
ADDRESS
TRDY#
(T)
(T)
DEVSEL#
LOCK
TARGET
BECOMES
LOCKED
MECHANISM
AVAILABLE
UPON FIRST
ACCESS
(I) = DRIVEN BY INITIATOR
(T) = DRIVEN BY TARGET
BUS
IDLE
LOCK MECHANISM
AVAILABLE
LOCK ESTABLISHED
LOCK MAINTAINED
Table 43. Possible Combinations of FRAME# and IRDY#
FRAME#
deasserted deasserted Bus Idle
deasserted asserted The initiator is ready to complete the last data transfer of a transaction.
IRDY#
Description
asserted
deasserted An Initiator has a transaction in progress but is not able to complete the data transfer on this
clock.
asserted
asserted
An initiator has a transaction in progress and is able to complete a data transfer.
112
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