INDEX
PRELIMINARY
MX8335
RAMBUS CLOCK GENERATOR
FEATURES
• Clock generator for Rambus
TM
Channel
• Provide two Rambus interface-level outputs with
frequency from 200MHz to 267MHz
• Multiplication ratio 40/3 allows low cost crystal used
as frequency reference
• Provide chip output enable pin (EN) for tri-state
control
• 3.0V~3.6V power supply range
• 8-pin SOP (150mil) package
GENERAL DESCRIPTION
The MX8335 is a clock synthesizer chip for Rambus
channel. With an advanced phase locked loop technol-
ogy, the MX8335 provides two clock outputs (CLKOUT)
with frequency ranging from 200MHz to 267MHz. The
desired high speed clock is the 40/3 Multiplication ratio
of a reference input clock. The reference clock could
be supplied by an external crystal ranging from 15MHz
to 20MHz.
PIN CONFIGURATIONS
8-PIN SOP
PIN DESCRIPTION
SYMBOL PIN TYPE PIN NUMBER
EN
I
1
DESCRIPTION
When high, CLKOUTs are
enabled.
When low, CLKOUTs are
tristate.
EN
XO
XI
GND
1
2
3
4
8
VDD
CLKOUT
GND
CLKOUT
XO
XI
GND
CLKOUT O
GND
CLKOUT O
VDD
O
I
2
3
4
5
6
7
8
MX8335
7
6
5
With an internal pull high
resistor.
Crystal output pin.
Crystal input pin.
Ground.
200-267Mhz output.
Ground.
200-267Mhz output.
Power supply.
BLOCK DIAGRAM
DIVIDER
÷40
200MHz~267MHz
XO
15MHz~20MHz
XI
OSC
DIVIDER
÷3
PFC
CHARGE
PUMP
VCO
200MHz~267MHz
VDD
LOOP
FILTER
EN
Rambus and
P/N: PM0334
are trademarks of Rambus Inc.
REV. 1.4, APR. 15, 1996
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INDEX
MX8335
FUNCTIONAL DESCRIPTION
The Rambus clock generator is an integrated circuit of the
phase locked loop frequency synthesizer. It provides two
clock outputs for dual Rambus channel systems.
As shown in the block diagram, a phase locked loop
consists of dividers, phase frequency comparator(PFC),
charge pump, voltage controlled oscillator(VCO), and
loop filter. All components for PLL are integrated inside
the chip.
The dividers provide a fixed multiplication ratio 40/3.
When a reference clock, ranging from 15MHz to 20MHz
and either coming from crystal or external system refer-
ence, provides to PLL, the clock outputs should fall in the
range of 200-266.67MHz. Following examples shows the
relationship between input and output frequencies:
Reference Frequency
15MHz
18.75MHz
20MHz
Output Frequency
200MHz
250MHz
266.67MHz
OUTPUT CIRCUITRY
The clock source output is an open drain NMOS transis-
tor. The MX8335 enables and disables the NMOS
transistor alternately to drive the output load. The biasing
circuity for the MX8335 is shown in Fig. 1.
Output high voltage is determined by the Vt voltage. To
set a value for output low voltage, we must determine the
Rs and Rt values. Rt is determined by system operating
conditions, which may be ranging from 25 Ohms to 50
Ohms. Rs can be acquired by following equation:
Rs=(Rt x Vt/Vs)-(Rt+Ro)
Ro=On resistor of NMOS transistor
Vs=Signal Swing
For a system with Vt=2.5V, Rt=25 Ohms, Vs=1.4V, and
the MX8335's Ro=10 Ohms
Rs=(25 x 2.5/1.4)-(25+10)=9.6 Ohms
Cx capacitor can help equalize rising and falling rate. Rx
resistor helps decrease the rising time. Both Rx and Cx
depend on Rt+Rs and PCB layout. Typical values of Rx
and Cx for 25 Ohms line impedance are
∞
Ohms and 4pF
respectively. For 50 Ohms line impedance, Rx is about
100 Ohms, and Cx is about 2pF. Special care must be
taken during physical design of the output bias circuits
and the Rambus channel.
Rx, Cx, and Rs should be as close to the MX8335 as
possible, and are prefered to use SMD devices. The
signal path must be built using controlled impedence
transmission line technique.
FREQUENCY REFERENCE
The internal reference oscillator includes all passive
components required. A proper resonant crystal should
be connected between XO and XI. To minimize the noise
pick up, maintain short lead lengths between the crystal
and the MX8335 by soldering the crystal to the ground
plane. The lead length of the capacitor should be kept to
minimum to reduce noise suscepibility. The reference
clock can also be supplied by external clock signal. In this
case, the reference clock should connected to XI and XO
should be unconnected.
POWER SUPPLY CONDITIONING
Clock jittering is the undesirable variations in frequency
and phase of a clock source. In PLL, there are many clock
jitter sources, such as flicker and thermal noise in elec-
tronic element, electromagnetic coupling, power supply
noise, signal reflection, ground bounce, physical
vibration and variation in temperature and humidity.
Among these noise factors, the power supply noise and
ground bounce are most crucial and should be given
special care. To eliminate the supply noise, it is judicious
to decouple the power noise as shown in Fig. 1, where
C1=0.01uF, C2=0.1uF, and C3=47uF.
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