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Hardware Reference Manual 145
Intel
®
IXP2800 Network Processor
Intel XScale
®
Core
3.12.7.4 Address Space
The total address space is defined as 64 Mbytes, which is further divided into two segments of
32 Mbytes each. Two devices can be connect to this bus. If these peripheral devices have a density
of 256 Mbits (32 Mbytes) each, all the address space is going to be filled like a contiguous address
space. However, if a small capacity device is used (like a device of 4, 8, or 16 MBytes), there will
be a memory hole left in between these two devices. Figure 36 is a 4-Mbyte memory example.
Trying to read the space in between, you will get the repeating value for each 4-Mbyte location
3.12.7.5 Slowport Interfacing Topology
Figure 37 demonstrates one of the topologies used to connect to an 8-bit device. From the diagram,
we can observe that address is shifted out eight bits at a time and buffered into three 74F377 or
equivalent tri-state buffer devices in three consecutive clock cycles. These buffers also output
separately to form a 25-bit wide address bus to address the 8-bit devices. The data are expected to
be driven out after the address has been placed into the buffers.
There are two devices shown in Figure 37. The top one is the fix-timed device, while the lower
one, self-timing device. For the self-timing device, the access latency depends on the SP_ACK_L
responded back from this device.
Three extra signals, SP_CP, SP_OE_L, and SP_DIR, are added to pack and unpack the data when a
16-bit or 32-bit device is hooked up to Slowport. They are used for special application only as
described below.
Figure 36. Address Space Hole Diagram
A9705-01
3FFFFFFh
23FFFFFh
2000000h
03FFFFFh
0000000h
4 MB
4 MB