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126 Hardware Reference Manual
Intel
®
IXP2800 Network Processor
Intel XScale
®
Core
The Intel XScale
®
core coprocessor bus is not used in the IXP2800 Network Processors, therefore
all accesses are only through the Command Memory Bus.
Figure 27 shows the block diagram of the global bus connections to the gasket.
The gasket unit has the following features:
• Interrupts are sent to the Intel XScale
®
core via the gasket, with the interrupt controller
registers used for masking the interrupts.
• The gasket converts CMB reads and writes to CPP format.
• All the atomic operations are applied on SRAM and SCRATCH only, not DRAM.
• There is a stepping-stone sitting between the Intel XScale
®
core and the gasket. The Intel
XScale
®
core runs at 600 – 700 MHz. The gasket currently supports a 1:1 (IXP2800 Network
Processor) clock ratio. For a 2:1 ratio, the Command Push Pull bus will be running at ½ of the
frequency of the Intel XScale
®
core.
• In IXP2800 memory controllers, read after write ordering is enforced. There is no write after
read enforcement for the Intel XScale
®
core. The gasket will perform enforcement by
employing Content Addressable Memory (CAM) to detect a write to an address with read
pending. This only applies for writes to SRAM.
• The gasket CPP interface contains one command bus, one D_Push bus, one D_Pull bus, one
S_Push bus, and one S_Pull bus, each with a 32-bit data width.
A maximum four outstanding reads and four outstanding writes from the Intel XScale
®
core are
allowed.
Figure 27. Global Buses Connection to the Intel XScale
®
Core Gasket
A9697-03
Req
CAP CSR
Intel XScale
®
Core
Gasket
Local
CSR
CMD_BUS
DRAM_PUSH_BUS
SRAM_PULL_BUS
DRAM_PULL_BUS
SRAM_PUSH_BUS