BCM1250/BCM1125/BCM1125H
User Manual
10/21/02
B r o a d c o m C o r p o r a t i o n
Page
190
Section 8: PCI Bus and HyperTransport Fabric
Document
1250_1125-UM100CB-R
S e c t i o n 8 : P C I B u s a n d H y p e r Tr a n s p o r t
F a b r i c
I
NTRODUCTION
The PCI bus and HyperTransport (formerly called “Lightning Data Transport” or “LDT”) fabric provide the main
general expansion buses for the BCM1250 and BCM1125/H parts. PCI is the standard bus used for peripheral
connection in many systems and there are a wide variety of peripheral devices that connect to it.
HyperTransport (HT) is a high performance replacement for PCI on system boards, it uses a chain of
unidirectional point to point links to form a fabric of devices running at much higher data rate. The
HyperTransport devices use the standard PCI configuration process, so devices can be migrated from the PCI
to HyperTransport with few software changes. Initial devices available for the HyperTransport fabric include
bridges to PCI, PCI-X and AGP. The BCM1250 and BCM1125H have both HT and PCI interfaces, the
BCM1125 has only PCI.
The PCI interface conforms to revision 2.2 of the PCI standard. It is a 32 bit wide interface, can run up to 66MHz
and uses 3.3V signal levels (the interface is
not
5V tolerant). PCI special cycles, dual address cycles and the
LOCK protocol are not supported. The interface can act as either the host bridge or as a device configured and
controlled by another host. These are referred to as Host Mode and Device Mode. When operating in Host
Mode an internal arbiter can be used, providing support for four external PCI devices; alternatively an external
arbiter can be used to allow more external devices.
The HyperTransport interface conforms to revision 1.03 of the HyperTransport specification although to ensure
software backwards compatibility with the earlier implementation (based on 0.17 of the HT specification) there
are a few differences, mainly in the mechanisms for error reporting (this was not specified in the earlier version
of the standard). The HyperTransport transmit and receive clocks are usually the same frequency (the receive
link frequency may be lower than the transmit frequency), generated from the 100 MHz reference clock. The
HyperTransport transmit clock can be the reference clock x2, x3, x4, x5 or x6. The interface runs 8 bits wide
in each direction, with data sent on both edges of the clock. The HyperTransport interface runs as a host
bridge, and therefore is always on one end of the HyperTransport fabric. Support is provided for fabrics with
host bridges at each end.
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