
54
Hardware Design Guide
IXP28XX Network Processor
QDR SRAM
significant changes in results or may lead to non-working design altogether. That is why it is
strongly recommended that developers thoroughly simulate any new designs or modifications
before committing to any new modifications.
4.2
QDR Clocking Scheme
The controller drives out a pair of K clocks (K and K#), and a pair of C clocks (C and C#). The C
and C# clocks externally return to the controller for reading data.
Figure 21
shows the clock
diagram of the clocking scheme for a QDR interface driving four SRAM chips.
4.2.1
SRAM Controller Configurations
Each channel has enough address pins (24) to support up to 64 Mbytes of SRAM. The SRAM
controllers can directly generate multiple port enables (up to four pairs) to allow for depth
expansion; two pairs of pins are dedicated for port-enables. Smaller RAMs use fewer address
signals than the number provided to accommodate the largest RAMs, so some address pins – 23:20
– are configurable as either address or port-enable, based on the control status register (CSR)
setting, as shown in
Table 14
.
Note:
All of the SRAMs on a given channel must be the same size.
Figure 21.
Clocking Scheme for a QDR Interface Driving Four SRAMs
A9234-03
Clam-shelled SRAMS
Package Balls
Package Balls
Termination
Intel®
IXP2800
Network
Processor
C0IN/C0IN#
K0/K0#
K/K#
K/K#
C/C#
C/C#
C0/C0#
K1/K1#
C1/C1#
C0/C0#
C1/C1#
C1IN/C1IN#
Termination
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