SY87729LHI Micrel Inc, SY87729LHI Datasheet - Page 5

• Manufacturer Part Number: SY87729LHI
• Description: IC SYNTHESIZER FRACT 3.3V 32TQFP
• Manufacturer: Micrel Inc
• Type: Fractional Synthesizerr

Specifications of SY87729LHI

• Input: PECL
• Output: PECL
• Frequency - Max: 365MHz
• Voltage - Supply: 3.15 V ~ 3.45 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 32-TQFP Exposed Pad, 32-eTQFP, 32-HTQFP, 32-VQFP
• Frequency-max: 365MHz
• For Use With: 576-1404 - BOARD EVAL N SY87729 EXPERIMENT
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Micrel, Inc.
General
used in serial data streaming applications, where the
incoming data rate on a channel may vary, or where the
incoming data rate on a channel is unknown ahead of time.
generated even in the absence of any edges on the
corresponding input stream. Until now, designers had to
resort to sub-optimal solutions such as providing multiple
reference oscillators. Beyond the potential noise and EMI
issues, the designer has no way to future proof his circuit,
as it would prove near impossible to pre-provision all the
reference frequencies that might be needed after
deployment, yet are unknown at this time.
frequencies for common data streaming protocols, all from
one 27MHz reference. If any of these protocols include
overhead due to use of common digital wrappers, the
SY87729L still generates the exact frequency required,
including the overhead.
directly, The SY87729L also generate reference frequencies
for Micrel’s SY87721L CDR/CMU, such that it will reliably
recover data at any rate between 28Mbps and 2,700Mbps
without any gaps.
configuration that describes the desired output reference
frequency. All common microcontrollers support this
MicroWire™ interface. Those microcontrollers that don’t
support this interface in hardware can easily emulate the
interface in firmware.
generates, are divided into three classes. First, the sets of
frequencies that match a particular data streaming protocol
are in the “protocol” category. Second, the set of frequencies
that are guaranteed to be near enough to any arbitrary data
rate such that the SY87721L will lock are in the “picket
fence” category. Third, the set of frequencies that do not fit
into either of the first two categories is in the third category,
frequencies through two tandem PLL circuits. The first PLL
uses a modified fractional-N approach to generate a rational
ratio frequency. This PLL is capable of generating all protocol
data rates, except for those that include FEC or digital
wrapper overhead. A second, more traditional P/Q
synthesizer optionally adjusts the output frequency of the
first, fractional-N synthesizer, to accommodate these FEC
or digital wrapper data rates.
data from a microcontroller to SY87729L. This simple
interface consists of an active high chip select, a serial
clock (2MHz or less) and a serial data input. Each clock
cycle one bit of configuration data transfers to SY87729L.
M9999-062807
[e-mail] or [phone]
DESCRIPTION
The SY87729L AnyClock
In these situations, a valid output stream must still be
The SY87729L solves this problem by generating exact
Besides generating reference rates for common protocols
A simple 3-wire MicroWire™ bit-serial interface loads a
The large set of possible frequencies that the SY87729L
The SY87729L generates these important reference
The bit serial interface conveys 32 bits of configuration
®
Fractional-N Synthesizer is
5
Circuit Description
shown in Figure 1.
one count. That is, if one divider is set to divide by P = 5,
then the other divider divides by P-1 = 4 . The mux chooses
between the two based on the control circuit.
input reference edge is used. Only those output edges that
are nearest to an input edge get fed back to the phase-
frequency comparator. In addition, the nearest output edges
are chosen in such a way that the net offset, over a number
of edges, zeroes out. It is the control circuit’s job to drive
the mux such that only the “correct” edges get fed back.
should be, for example, 5 times the input frequency, then P
is set to 5, and the control circuit sets the mux to only feed
back the output of the P divider.
the input frequency, then the control circuit alternates evenly
between the P and the P-1 divider output. For every two
input edges (one to compare against P, and another to
compare against P-1), you will get 5 + 4 output edges,
yielding an output frequency 9/2 the input frequency.
from input to output frequency, the control circuit determines
the fractional part. By mixing the output of the P and P-1
dividers correctly, the control circuit can fashion any output
frequency from P-1 times the input to P times the input, as
long as that ratio can be expressed as a ratio of integers.
The heart of SY87729L is its fractional-N synthesizer, as
The two dividers in the feedback path always differ by
The idea behind the fractional-N approach is that every
In the above fractional-N circuit, if the output frequency
If the output frequency should be, for example, 4
Whereas P sets the integer part of the multiplication factor
Figure 1. Fractional-N Synthesizer Architecture
Frequency
Reference
(f
Input
REF
)
Frequency
Detector/
Charge
Phase-
Pump
Control
Mux
Loop Filter
P-1
P
VCO
Output
Frequency
(f
AnyClock
SY87729L
FNOUT
1
/
2
)
times
®