ACS8530 datasheet(21/152 Pages) SEMTECH

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ADVANCED COMMUNICATIONS

FINAL

DATASHEET

Revision 3.02/November 2005 © Semtech Corp.

Page 21

www.semtech.com

ACS8530 SETS

The structure of the T0 and T4 PLLs are shown later in

Figure 11 in the section on output clock ports. That

section also details how the DPLLs and particular output

frequencies are configured. The following sections detail

some component parts of the DPLL.

TO DPLL Automatic Bandwidth Controls

In Automatic Bandwidth Selection mode (Reg. 3B Bit 7),

the T0 DPLL bandwidth setting is selected automatically

from the Acquisition Bandwidth or Locked Bandwidth

configurations programmed in cnfg_T0_DPLL_acq_bw

Reg. 69 and cnfg_T0_DPLL_locked_bw Reg. 67

respectively. If this mode is not selected, the DPLL

acquires and locks using only the bandwidth set by

Reg. 67.

Phase Detectors

A Phase and Frequency detector is used to compare input

and feedback clocks. This operates at input frequencies

up to 77.76 MHz. The whole DPLL can operate at spot

frequencies from 2 kHz up to 77.76 MHz (155.52 MHz is

internally divided down to 77.76 MHz). A common

arrangement however is to use Lock8k mode (See

Reg. 22 to 2D, Bit 6) where all input frequencies are

divided down to 8 kHz internally. Marginally better MTIE

figures may be possible in direct lock mode due to more

regular phase updates. This direct locking capability is

one of the unique features of the ACS8530.

A patented multi-phase detector is used in order to give

an infinitesimally small input phase resolution combined

with large jitter tolerance. The following phase detectors

are used:

Phase and frequency detector (±360 deg or

±180deg range)

An Early/ Late Phase detector for fine resolution

A multi-cycle phase detector for large input jitter

tolerance (up to 8191 UI), which captures and

remembers phase differences of many cycles

between input and feedback clocks.

The phase detectors can be configured to be immune to

occasional missing input clock pulses by using nearest

edge detection (±180 deg capture) or the normal

± 360 deg phase capture range which gives frequency

locking. The device will automatically switch to nearest

edge locking when the multi-UI phase detector is not

enabled, and the other phase detectors have detected

that phase lock has been achieved. It is possible to

disable the selection of nearest edge locking via Reg. 03

Bit 6 set to 1. In this setting, frequency locking will always

be enabled.

The balance between the first two types of phase detector

employed can be adjusted via registers 6A to 6D. The

default settings should be sufficient for all modes.

Adjustment of these settings affects only small signal

overshoot and bandwidth.

The multi-cycle phase detector is enabled via Reg. 74, Bit

6 set to 1 and the range is set in exponentially increasing

steps from ±1 UI, 3 UI, 7 UI, 15 UI … up to 8191 UI via

Reg. 74, Bits [3:0]. When this detector is enabled it keeps

a track of the correct phase position over many cycles of

phase difference to give excellent jitter tolerance. This

provides an alternative to switching to Lock8k mode as a

method of achieving high jitter tolerance.

An additional control (Reg. 74 Bit 5) enables the multi-

phase detector value to be used in the final phase value

as part of the DPLL loop. When enabled by setting High,

the multi cycle phase value will be used in the loop and

gives faster pull in (but more overshoot). The

characteristics of the loop will be similar to Lock8k mode

where again large input phase differences contribute to

the loop dynamics. Setting the bit Low only uses a max

figure of 360 degrees in the loop and will give slower pull-

in but gives less overshoot. The final phase position that

the loop has to pull in to is still tracked and remembered

by the multi-cycle phase detector in either case.

Phase Lock/Loss Detection

Phase lock/loss detection is handled in several ways.

Phase loss can be triggered from:

The fine phase lock detector, which measures the

phase between input and feedback clock

The coarse phase lock detector, which monitors whole

cycle slips

Detection that the DPLL is at min or max frequency

Detection of no activity on the input.

Each of these sources of phase loss indication is

individually enabled via register bits (see Reg. 73, 74 and

4D). Phase lock or lost is used to determine whether to

switch to nearest edge locking and whether to use

Acquisition or Locked bandwidth settings for the DPLL.

Acquisition bandwidth is used for faster pull in from an

unlocked state.

The coarse phase lock detector detects phase differences

of n cycles between input and feedback clocks, where n is

set by Reg. 74, Bits 3:0; the same register that is used for

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