CY7C1516AV18, CY7C1527AV18
CY7C1518AV18, CY7C1520AV18
Document Number: 001-06982 Rev. *C
Page 9 of 30
current data. The SRAM does this by bypassing the memory
array and reading the data from the registers.
Depth Expansion
Depth expansion requires replicating the LD control signal for
each bank. All other control signals can be common between
banks as appropriate.
Programmable Impedance
An external resistor, RQ, must be connected between the ZQ pin
on the SRAM and VSS to allow the SRAM to adjust its output
driver impedance. The value of RQ must be 5x the value of the
intended line impedance driven by the SRAM. The allowable
range of RQ to guarantee impedance matching with a tolerance
of ±15% is between 175
Ω and 350Ω, with V
DDQ =1.5V. The
output impedance is adjusted every 1024 cycles upon power up
to account for drifts in supply voltage and temperature.
Echo Clocks
Echo clocks are provided on the DDR-II to simplify data capture
on high-speed systems. Two echo clocks are generated by the
DDR-II. CQ is referenced with respect to C and CQ is referenced
with respect to C. These are free-running clocks and are
synchronized to the output clock of the DDR-II. In single clock
mode, CQ is generated with respect to K and CQ is generated
with respect to K. The timing for the echo clocks is shown in the
Switching Characteristics on page 23.
DLL
These chips use a DLL that is designed to function between 120
MHz and the specified maximum clock frequency. During power
up, when the DOFF is tied HIGH, the DLL is locked after 1024
cycles of stable clock. The DLL can also be reset by slowing or
stopping the input clock K and K for a minimum of 30 ns.
However, it is not necessary to reset the DLL to lock to the
desired frequency. The DLL automatically locks 1024 clock
cycles after a stable clock is presented. The DLL may be
disabled by applying ground to the DOFF pin. When the DLL is
turned off, the device behaves in DDR-I mode (with one cycle
latency and a longer access time). For information refer to the
application note DLL Considerations in QDRII™/DDRII.
Application Example
Figure 1 shows two DDR-II used in an application.
Figure 1. Application Example
Vterm = 0.75V
Vterm = 0.75V
R = 50
ohms
R = 250
ohms
LD#
C C#
R/W#
DQ
A
K
LD#
C C#
R/W#
DQ
A
K
SRAM#1
SRAM#2
R
= 250ohms
BUS
MASTER
(CPU
or
ASIC)
DQ
Addresses
Cycle Start#
R/W#
Return CLK
Source CLK
Return CLK#
Source CLK#
Echo Clock1/Echo Clock#1
Echo Clock2/Echo Clock#2
ZQ
CQ/CQ#
K#
ZQ
CQ/CQ#
K#
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