User manual

Hardware Reference Manual 221
Intel
®
IXP2800 Network Processor
SRAM Interface
Other microcode rules:
• All access to atomic variables should be through read-modify-write instructions.
• If the flow must know that a write is completed (actually in the SRAM itself), follow the write
with a read to the same address. The write is guaranteed to be complete when the read data has
been returned to the Microengine.
• With the exception of initialization, never do write commands to the first three longwords of a
queue_descriptor data structure (these are the longwords that hold head, tail, and count).
All accesses to this data must be through the Q commands.
• To initialize the Q_array registers, perform a memory write of at least three longwords,
followed by a memory read to the same address (to guarantee that the write completed). Then,
for each entry in the
Q_array, perform a read_q_descriptor_head followed by a
read_q_descriptor_other using the address of the same three longwords.
6.8 Coprocessor Mode
Each SRAM controller may interface to an external coprocessor through its standard QDR
interface. This interface allows for the cohabitation of both SRAM devices and coprocessors
operating on the same bus, and the coprocessor behaves as a memory-mapped device on the SRAM
bus. Figure 82 is a simplified block diagram of the SRAM controller. Figure 82 shows the
connection to a coprocessor through a standard QDR interface.
Note: Most coprocessors do not need a large number of address bits — connect as many bits of A
n as
required by the coprocessor.
Figure 82. Connection to a Coprocessor Though Standard QDR Interface
A9746-01
Push Data
FIFO
Read Cmd
FIFO
Write Cmd
FIFO
Pull Data
FIFO
Pin
Control
State
Mechanics
Coprocessor
Internal
Bus
Control
State
Mechanics
SRAM Push Bus
SRAM Cmd Bus
SRAM Pull Bus
SRAM_Control Coproessor
Qn[17:0]
Read
Data
Read
Address
Write
Address
Write
Data
RPE_Ln[1:0]
An[x:0]
BWEn[1:0]
WPE_Ln[1:0]
Dn[17:0]