Cypress CY7C1527KV18 Manual De Usuario

Document Number: 001-00437 Rev. *E

Page 9 of 30

An external resistor, RQ, must be connected between the ZQ pin

on the SRAM and V

SS 

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 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 synchro-

nized 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 

These chips use a PLL that is designed to function between 

120 MHz and the specified maximum clock frequency. During

power up, when the DOFF is tied HIGH, the PLL is locked after

20 

μs of stable clock. The PLL 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 PLL to lock to the

desired frequency. The PLL automatically locks 20 

μs after a

stable clock is presented. The PLL may be disabled by applying

ground to the DOFF pin. When the PLL is turned off, the device

behaves in DDR-I mode (with one cycle latency and a longer

access time).

 shows two DDR-II used in an application.

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#