High-Precision Clock Generators

with Integrated VCXO

8

_______________________________________________________________________________________

N1 and N2, respectively. CR5 and CR6 are the control

function registers for output enabling, reference clock

selection, and activation of the clock monitor and the

holdover function. CR7 contains the status of clock

monitor, holdover, and PLL locking. The addresses of

the eight registers are shown in Table 4. Tables 5

through 10 show the register maps.

Output Buffers

Three different output formats (LVPECL, HSTL, and

LVDS) are available. Each output contains its own

frequency divider. All the output clocks align to their

coincident rising edges. After changing the dividing

ratio, the output clocks complete the current cycle and

stay logic-low until the rising edges of the newly divided

clock. When CR5[7] is high, the MAX9450/MAX9451/

MAX9452 set all the outputs to logic-low. Setting the

bits CR5[6] and CR5[5] properly enables and disables

the outputs individually; see Table 8. A disabled output

is always in high impedance. At the receiver end, the

two cables or PCB traces can be terminated as shown

in Figure 1.

The VCXO output is divided down before driving the out-

put buffers. Program the dividing factor through the serial

interface. The MAX9450/MAX9451/MAX9452 feature two

output dividers DIV0 and DIV1 (see the Functional

Diagram). DIV0 drives OUT0 and either DIV0 or DIV1 can

drive OUT1. CR6[2] sets which divider output drives

OUT1. This function allows for programming OUT1 and

OUT0 to different frequencies.

Reference Clock Inputs

The MAX9450/MAX9451/MAX9452 feature two “any-

thing” differential clock inputs. “Anything” means that

the inputs take any differential signals, such as CML,

LVDS, LVPECL, or HSTL. The inputs can also take a

single-ended input. For example, with LVCMOS refer-

ence inputs, connect the inputs to the positive pins

INn+ and connect the negative pins INn- to a reference

voltage of VDD - 1.32V. See Figure 2.

Setting CR5[4] and CR6[3] selects the input reference.

Failure detection and revert function apply only to IN0

and IN1. Also, SEL0 and SEL1 or CR5[3:2] can disable

the corresponding inputs. See Table 2.

Frequency Selection and Programming

The output frequency at CLKn, (n = 0, 1) is determined by

the reference clock and the dividing factors M, Ni (i = 0, 1),

and P, shown in the following equation:

ff

M

Ni P

CLKn

REF

=×

×

LVCMOS CLK OUTPUT

VREF = VDD - 1.32V

ANYTHING INPUT

Figure 2. Connecting LVCMOS Output to LVPECL Input

Z = 50

Ω

LVPECL

OUTPUT

127

Ω

127

Ω

3.3V

83

Ω

83

Ω

LVPECL

INPUT

(A) LVPECL DC-COUPLING

Z = 50

Ω

HSTL

OUTPUT

50

Ω

50

Ω

HSTL

INPUT

(C) HSTL DC-COUPLING

(B) LVDS COUPLING

Z = 50

Ω

LVDS

OUTPUT

100

Ω

LVDS

INPUT

Figure 1. DC LVPECL, LVDS, and HSTL Termination