Revision 1.02 – June 27, 2006
S5935 – PCI Product
PCI BUS INTERFACE
Data Book
A data phase consists of at least one PCI clock.
FRAME# is deasserted to indicate that the final data
phase of a PCI cycle is occurring. Wait states may be
added to any data phase (each wait state is one PCI
clock).
This section describes the various events which occur
on the S5935 PCI bus interface. Since the S5935 con-
troller functions as both a target (slave) and an initiator
(master), signal timing detail is given for both situa-
tions this Section presents the signal relationships
involved in performing basic read or write transfers on
the PCI bus and also describes the different ways
these cycles may complete.
The PCI bus command presented on the C/BE[3:0]#
pins during the address phase can represent 16 possi-
ble states. Table 1 lists the PCI commands and
identifies those which are supported by the S5935
controller as a target and those which may be pro-
duced by the S5935 controller as an initiator. A “Yes”
in the “Supported As Target” column in Table 1 indi-
cates the S5935 controller asserts the signal
DEVSEL# when that command is issued along with
the appropriate PCI address. Two commands are sup-
ported by the S5935 controller as an initiator: Memory
Read and Memory Write.
PCI BUS TRANSACTIONS
Because the PCI bus has multiplexed address/data
pins, AD[31:0], each PCI bus transaction consists of
two phases: Address and Data. An address phase is
defined by the clock period when the signal FRAME#
transitions from inactive (high) to active (low). During
the address phase, a bus command is also driven by
the initiator on signal pins C/BE[3:0]#. If the command
indicates a PCI read, the clock cycle following the
address phase is used to perform a “bus turn-around”
cycle. A turn-around cycle is a clock period in which
the AD bus is not driven by the initiator or the target
device. This is used to avoid PCI bus contention. For a
write command, a turn-around cycle is not needed,
and the bus goes directly from the address phase to
the data phase.
The completion or termination of a PCI cycle can be
signaled in several ways. In most cases, the comple-
tion of the final data phase is indicated by the
assertion of ready signals from both the target
(TRDY#) and initiator (IRDY#) while FRAME# is inac-
tive. In some cases, the target is not be able to
continue or support a burst transfer and asserts the
STOP# signal. This is referred to as a target discon-
nect. There are also cases where an addressed
device does not exist, and the signal DEVSEL# never
becomes active. When no DEVSEL# is asserted in
response to a PCI cycle, the initiator is responsible for
ending the cycle. This is referred to as a master abort.
The bus is returned to the idle phase when both
FRAME# and IRDY# are deasserted.
All PCI bus transactions consist of an address phase
(described above), followed by one or more data
phases. The address phase is only one PCI clock long
and the bus cycle information (address and command)
is latched internally by the S5935. The number of data
phases depends on how many data transfers are
desired or are possible with a given initiator-target pair.
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