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336 Hardware Reference Manual
Intel
®
IXP2800 Network Processor
PCI Unit
Note: The IXP2800/IXP2850 always disconnects after transferring 16-bytes for DRAM target reads. The
PCI core will also disconnect at a 64-byte address boundary.
The PCI core resets the read FIFO before issuing a memory read data request on FBus. The PCI
core will disconnect at the 64-byte address boundary.
9.3.4 Mailbox and Doorbell Registers
Mailbox and Doorbell registers provide hardware support for communication between the Intel
XScale
®
core and a device on the PCI Bus.
Four mailbox registers are provided so that messages can be passed between the Intel XScale
®
core
and a PCI device. All four registers are 32 bits and can be read and written with byte resolution
from both the Intel XScale
®
core and PCI. How the registers are used is application dependent and
the messages are not used internally by the PCI Unit in any way. The mailbox registers are often
used with the Doorbell interrupts.
Doorbell interrupts provide an efficient method of generating an interrupt as well as encoding the
purpose of the interrupt. The PCI Unit supports an Intel XScale
®
core Doorbell register that is used
by a PCI device to generate an Intel XScale
®
core FIQ and a separate PCI Doorbell register that is
used by the Intel XScale
®
core to generate a PCI interrupt. A source generating the Doorbell
interrupt can write a software defined bitmap to the register to indicate a specific purpose. This
bitmap is translated into a single interrupt signal to the destination (either a PCI interrupt or a
IXP2800 Network Processor interrupt). When an interrupt is received, the Doorbell registers can
be read and the bit mask can be interpreted. If a larger bit mask is required than that is provided by
the Doorbell register, the Mailbox registers can be used to pass up to four 32-bit blocks of data.
Figure 123. Example of Target Read from DRAM Using 64-Byte Burst
A9770-02
11111111
11111111
11111111
11111111
11111111
11111111
11111111
11111111
00000000
00000000
00000000
00000000
Memory Transfer Internal
Bus Data
PCI Bus
Byte Lane
Swap
Slave Read Burst from memory
Starting address = 0x0
Transfer Size - 32 bytes
Byte EnablesAddress Size
Byte Enables
16 Byte
0x0
Address
0x00
0x10
0x10
Disconnect