LMX2346 datasheet(21/27 Pages) NSC

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1.0 Functional Description (Continued)

1.7 MICROWIRE INTERFACE

The programmable register set is accessed via the

Microwire serial interface. The interface is comprised of

three signal pins: CLOCK, DATA, and LE (Latch Enable).

Serial data is clocked in from DATA on the rising edge of

CLOCK, into an 18-bit shift register. The serial data is

clocked in MSB first. The last bit of data decodes the internal

the shift register is loaded into one of the two appropriate

latches based on the address bit. A complete programming

description is provided in Section 2.0.

1.8 MULTI-FUNCTION OUTPUT

The LMX2346/7 LD pin is a multi-function output that can be

configured as a digital lock detect, an analog lock detect, as

well as monitor the output of the reference divider, or feed-

back divider circuits. The LD_OUT control word is used to

select the desired output function. When the PLL is in power-

down mode, the LD output is always set to a high imped-

ance. A complete programming description of the multi-

function output is provided in Section 2.2.4.

1.8.1 Analog Lock Detect

When LD_OUT = 1, an analog lock detect status generated

from the phase detector is available on the LD output pin.

The lock detect output goes to high impedance when the

charge pump is inactive. It goes low when the charge pump

is active during a comparison cycle. The analog lock detect

signal output is an open drain configuration.

1.8.2 Digital Lock Detect

When LD_OUT = 0, a digital lock detect status is available

on the LD output pin. The digital lock detect filter compares

the phase difference of the inputs from the phase detector to

a RC generated delay of approximately 15 ns. To enter the

locked state (LD = High), the phase error must be less than

the 15 ns RC delay for 5 consecutive reference cycles. Once

in lock, the RC delay is changed to approximately 30 ns. To

exit the locked state, the phase error must be greater than

the 30 ns RC delay. A flow chart of the digital lock detect filter

follows.

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LMX2346 Datasheet(PDF) 21 Page - National Semiconductor (TI)