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Texas Instruments Selection and Specification of Crystals for TI's IEEE 1394 Physical Layers Application notes
Selection and Specification
of Crystals for Texas
Instruments IEEE 1394
Physical Layers
Application
Report
2000
Mixed-Signal Products
SLLA051
IMPORTANT NOTICE
Texas Instruments and its subsidiaries (TI) reserve the right to make changes to their products or to discontinue
any product or service without notice, and advise customers to obtain the latest version of relevant information
to verify, before placing orders, that information being relied on is current and complete. All products are sold
subject to the terms and conditions of sale supplied at the time of order acknowledgement, including those
pertaining to warranty, patent infringement, and limitation of liability.
TI warrants performance of its semiconductor products to the specifications applicable at the time of sale in
accordance with TI’s standard warranty. Testing and other quality control techniques are utilized to the extent
TI deems necessary to support this warranty. Specific testing of all parameters of each device is not necessarily
performed, except those mandated by government requirements.
CERTAIN APPLICATIONS USING SEMICONDUCTOR PRODUCTS MAY INVOLVE POTENTIAL RISKS OF
DEATH, PERSONAL INJURY, OR SEVERE PROPERTY OR ENVIRONMENTAL DAMAGE (“CRITICAL
APPLICATIONS”). TI SEMICONDUCTOR PRODUCTS ARE NOT DESIGNED, AUTHORIZED, OR
WARRANTED TO BE SUITABLE FOR USE IN LIFE-SUPPORT DEVICES OR SYSTEMS OR OTHER
CRITICAL APPLICATIONS. INCLUSION OF TI PRODUCTS IN SUCH APPLICATIONS IS UNDERSTOOD TO
BE FULLY AT THE CUSTOMER’S RISK.
In order to minimize risks associated with the customer’s applications, adequate design and operating
safeguards must be provided by the customer to minimize inherent or procedural hazards.
TI assumes no liability for applications assistance or customer product design. TI does not warrant or represent
that any license, either express or implied, is granted under any patent right, copyright, mask work right, or other
intellectual property right of TI covering or relating to any combination, machine, or process in which such
semiconductor products or services might be or are used. TI’s publication of information regarding any third
party’s products or services does not constitute TI’s approval, warranty or endorsement thereof.
Copyright  2000, Texas Instruments Incorporated
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2 Crystal Frequency of Oscillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1 Crystal Oscillation Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Crystal Circuit Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3 Frequency Tolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4 Temperature Tolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5 Aging (Long-Term Stability or Drift) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.6 Load Capacitance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.7 Equivalent Series Resistance (ESR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.8 Circuit Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
2
2
2
2
2
3
4
5
3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5 Product Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1 Related Documentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 World Wide Web . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3 Email . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
7
7
7
List of Figures
1 Crystal Equivalent Loading Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Crystal Frequency Dependence on Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3 Example Crystal Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Selection and Specification of Crystals for Texas Instruments IEEE 1394 Physical Layers
iii
iv
SLLA051
Selection and Specification of Crystals for Texas Instruments
IEEE 1394 Physical Layers
Burke Henehan
ABSTRACT
To comply with the IEEE 1394 standard, each node in a 1394 bus must have a system
clock of 49.152 MHz ±100 parts per million (ppm) (approximately 49.1471 MHz to
49.1569 MHz). For Texas Instruments (TI) 1394 physical layers, this clock is derived
from an external crystal with a frequency of 24.5760 MHz. An accurate clock requires
careful selection and specification of the crystal used to supply the clock, along with the
layout of the crystal circuit. This application report describes an appropriate specification
for a crystal, and includes recommendations for layout and capacitor selection.
1 Introduction
TI physical layers (PHYs) may use an external 24.5760-MHz crystal connected
between the XI and XO pins on the PHY to provide the PHY clock. The clock from
the crystal input must be accurate within ±100 ppm for the PHYs to function
correctly. This frequency tolerance—required by the IEEE 1394 standard for the
PHY clocks on each node—must be maintained over variations introduced over
production runs of boards and the environment in which the boards operate.
Every board must have a system clock (SCLK generated by the PHY) within ±100
ppm of 49.152 MHz (49.1471 MHz to 49.1569 MHz) to comply with the 1394
standard. If adjacent nodes differ by more than 200 ppm (one 100 ppm, the other
–100 ppm), long packets sent across the 1394 bus may be corrupted, with the
final bits of the packet being lost, causing a packet data CRC error. TI PHYs are
designed with maximum margins, but the 1394 limits must still be observed. The
following are typical specifications for crystals used with TI physical layers, and
some recommendations for implementation. Points discussed include:
a. Crystal frequency
b. Crystal mode of operation
c. Crystal circuit type
d. Frequency tolerance
e. Temperature tolerance
f. Aging
g. Load capacitance
h. Load capacitors tolerance
i. Maximum equivalent series resistance
j. Crystal and load capacitors layout
k. Testing
TI is a trademark of Texas Instruments Incorporated.
1
Crystal Frequency of Oscillation
2 Crystal Frequency of Oscillation
The frequency of oscillation for the crystal should be specified as 24.5760 MHz.
2.1
Crystal Oscillation Mode
The oscillation mode of operation for the crystal should be specified as
fundamental mode. This simplifies the resonant circuit that must be designed for
the crystal.
2.2
Crystal Circuit Type
The type of circuit for the crystal should be specified as parallel resonance. This
type of crystal is more precise and is needed to keep all nodes in a network to
within ±100 ppm. The frequency of oscillation for parallel resonance crystal
oscillator circuits is also dependent on the load capacitance presented to the
crystal. See load capacitance below for more information.
2.3
Frequency Tolerance
Frequency tolerance is the maximum allowable deviation from nominal
frequency at a specified temperature, usually 25°C. The tolerance of the crystal
frequency over the manufacturing process should be specified at approximately
±30 ppm. The total tolerance from the crystal, the load capacitors, the capacitive
load of the board, the capacitive load of the PHY pins, variation over temperature,
variation with gain, and the circuitry of the PHY must be less than ±100 ppm. For
this reason the total tolerance specified for the crystal must be less than 100 ppm.
TI currently recommends a crystal frequency tolerance of ±30 ppm.
2.4
Temperature Tolerance
Temperature tolerance is the maximum allowable deviation, from the frequency
at room temperature, over a specified temperature range. The tolerance of the
crystal frequency over temperature should be approximately ±30 ppm. Again, the
total tolerance from the crystal, the load capacitors, the capacitive load of the
board, the capacitive load of the PHY pins, variation over temperature, variation
with aging, and the circuitry of the PHY must be less than ±100 ppm. For this
reason the tolerance specified for the crystal must be less than 100 ppm. TI
currently recommends a crystal frequency tolerance of ±30 ppm.
2.5
Aging (Long-Term Stability or Drift)
Aging is the gradual change of a crystal’s frequency of oscillation over time. There
are many causes, including mass accumulation on the crystal due to
contamination and stress relief or buildup on the mechanical structures around
the quartz and the electrodes. Aging is typically specified as ppm per year value
and so must be accounted for over the expected lifetime of the end equipment.
The crystals used on TI lab test boards have an aging specification of ±5 ppm per
year.
These values can be traded off, for example the frequency tolerance may be
specified at ±40 ppm, the temperature may be specified at ±30 ppm, and aging
specified at ±3 ppm per year to give a total of ±79 ppm possible variation just due
to the crystal over three years.
2
SLLA051
Crystal Frequency of Oscillation
2.6
Load Capacitance
For parallel resonant mode type circuits, the load capacitance specified for the
crystal is very important since the frequency depends on the resonance of the
circuit, including the load capacitors. However, the total load capacitance is not
just the load capacitors, but will also be a function of the board layout and circuit.
The total load capacitance (CL) will affect the frequency the crystal oscillates at.
This means the load specified for the crystal includes the load capacitors (C9,
C10), the loading of the PHY pins (Cphy), and the loading of the board itself (Cbd).
See the schematic diagram in Figure 1.
C9
XI
24.576 MHz
X1
Cphy + Cbd
Is
C10
XO
Phy_GND
Figure 1. Crystal Equivalent Loading Schematic
To summarize: CL = [(C9×C10)/(C9+C10)] + Cphy + Cbd. Representative values
for Cphy are ~1 pF and for Cbd are about 0.8 pF per centimeter of board etch,
a typical board can have from 3 pF to 6 pF or more. The layouts of the TI
evaluation module boards show approximately 4 pF to 5 pF of board and PHY
capacitance. The capacitance of load capacitors C9 and C10 combine as
capacitors in series. This means that variation on each capacitor only adds about
one half to the total variation of the load capacitance. If the load capacitors were
22 pF ±5% and if both were exactly 22 pF the load they would present would be
(22 × 22)/(22+22) = 11 pF. If both were at the high limit:
[(22 × 1.05) × (22 × 1.05)] / [(22 × 1.05) + (22 × 1.05)] = 11.55 pF.
The difference, 11.55 – 11 = 0.55 pF is half of the 1.1 pF that is 5% of 22 pF. Since
the value of any capacitor is a random draw, the values will tend to moderate one
another, decreasing dependence on the variation introduced by the tolerance on
the load capacitors. For this reason 5% tolerance capacitors should be sufficient.
An example plot of how the frequency of oscillation is influenced by the load
capacitance is shown in Figure 2.
Selection and Specification of Crystals for Texas Instruments IEEE 1394 Physical Layers
3
Crystal Frequency of Oscillation
FREQUENCY
vs
LOAD CAPACITANCE
PPM From Specified Frequency
400
300
200
Frequency
100
0
–100
–200
–20
–10
0
10
20
Difference From Specified Load Capacitance – pF
Figure 2. Crystal Frequency Dependence on Loading
In this idealized case, if the load capacitance is exactly what is specified for the
crystal (difference = 0.0 pF), the unit oscillates at exactly the specified frequency
(PPM = 0). Note that the curve is steeper when the capacitive load is lower than
specified. However, another consideration is that if the load capacitance is too
high or the series equivalent resistance is too high (see below) the oscillator
within the PHY itself will not be able to drive the load and will not oscillate. For TI
PHYs the load capacitance recommended is from 10 pF to 15 pF with lower
values recommended, such as 10 or 12 pF. The crystal load should never be
specified as more than 20 pF as this may be more than the oscillator inside the
PHY can drive. If too large a value is used the oscillator may never start to oscillate
or may oscillate at a very low frequency, outputting an SCLK of ~32 MHz.
2.7
Equivalent Series Resistance (ESR)
Crystals may be modeled by a series circuit of an inductor, a capacitor, and a
resistor; along with a shunt capacitance in parallel with all three. The resistor is
known as the equivalent series resistance. The equivalent series resistance is a
figure of merit that is related to the loading the crystal and its resonance circuit
present to the PHY oscillator pins. For the purposes of specification, a smaller
maximum series resistance is better. For the TI PHY devices it is recommended
that the maximum equivalent series resistance of a crystal specified for a loading
of 15 pF should be less than 30 Ω. Lower capacitive load values should have
lower maximum ESRs specified. The specified maximum ESR should never be
more than 50 Ω.
4
SLLA051
Crystal Frequency of Oscillation
2.8
Circuit Layout
The layout of the crystal portion of the PHY circuit is important for getting the
correct frequency from the crystal, minimizing the noise introduced into the PHY
phase lock loop, and minimizing any emissions from the circuit. The crystal and
the two load capacitors should be considered a unit during layout. The crystal and
the load capacitors should be placed as close as possible to one another while
minimizing the loop area created by the combination of the three components.
Using smaller size capacitors may help in making this unit more compact.
Minimizing the loop area minimizes the effect of the resonant current (Is) that
flows in this resonant circuit. This layout unit (crystal and load capacitors) should
then be placed as close as possible to the PHY XI and XO pins to minimize etch
lengths. The silkscreen outlines for the TI TSBKOHCI403 EVM crystal and load
capacitors are shown on the left with the pad layouts for them shown on the right.
This layout is an example; other layouts may be done following the above
guidelines for even more compact layout with smaller loop areas.
Figure 3. Example Crystal Layout
Selection and Specification of Crystals for Texas Instruments IEEE 1394 Physical Layers
5
Summary
3 Summary
To summarize the recommendations:
• Crystal frequency: 24.5760 MHz
• Crystal mode of operation: fundamental
• Crystal circuit type: parallel resonance
• Frequency tolerance at 25°C: ≤ ±30 ppm
• Frequency stability over temperature: ≤ ±30 ppm
• Aging: ≤ ±5 ppm/year
• Load capacitance: [parallel (pF)]: 12 pF
• Load capacitors tolerance ≤ ±5%
• Maximum equivalent series resistance: ≤ 30 Ω
• Crystal and load capacitors placed as close as possible together as a unit
• Crystal and load capacitors unit placed as close as possible to PHY XI and
XO pins
• Test the frequency of SCLK output from the PHY and iterate on the load
capacitance to achieve a frequency within the tolerance specified for the
crystal.
4 References
•
•
•
6
Specifying Crystals for use in VCXOs and TCXOs for Wireless Designs, by
James Northcutt on Fox Electronics web page:
http://foxonline.com/tech3031.htm
Document link labeled In order to use a crystal unit under crystal units link on
KSS Kinseki web page (document in English):
http://www.kinseki.co.jp/eng/product.html
Electronic Engineers’ Handbook by Donald G. Fink and Donald Christiansen,
Sections 7 and 13.
SLLA051
Product Support
5 Product Support
The following sources provide related product support information.
5.1
Related Documentation
Further information and data sheets can be obtained via the Internet at:
http://www.ti.com/sc/1394.
Application reports can be obtained via the Internet at:
http://www.ti.com/sc/docs/psheets/app msp.htm.
The following documents are available via links from the TI 1394 external web
page:
• Data sheets for all TI 1394 devices
• Application notes for TI 1394 devices
• Errata list for all TI 1394 devices
• Information on designer kits
The IEEE 1394-1995 standard is available for purchase at:
http://standards.ieee.org/catalog/index.html
5.2
World Wide Web
Our World Wide Web site at www.ti.com contains the most up to data product
information, revisions, and additions. Users registering with TI&ME can build
custom information pages and receive new production updates automatically via
email
The URL specifically for the TI 1394 external web site is
http://www.ti.com/sc/1394. On this page, one can subscribe to 1394Times, which
periodically updates subscribers on events, articles, products, and other news
regarding 1394 developments.
5.3
Email
For technical issues or clarification on products, send a detailed email to:
sc-infomaster@ti.com.
Selection and Specification of Crystals for Texas Instruments IEEE 1394 Physical Layers
7
8
SLLA051
IMPORTANT NOTICE
Texas Instruments and its subsidiaries (TI) reserve the right to make changes to their products or to discontinue
any product or service without notice, and advise customers to obtain the latest version of relevant information
to verify, before placing orders, that information being relied on is current and complete. All products are sold
subject to the terms and conditions of sale supplied at the time of order acknowledgement, including those
pertaining to warranty, patent infringement, and limitation of liability.
TI warrants performance of its semiconductor products to the specifications applicable at the time of sale in
accordance with TI’s standard warranty. Testing and other quality control techniques are utilized to the extent
TI deems necessary to support this warranty. Specific testing of all parameters of each device is not necessarily
performed, except those mandated by government requirements.
CERTAIN APPLICATIONS USING SEMICONDUCTOR PRODUCTS MAY INVOLVE POTENTIAL RISKS OF
DEATH, PERSONAL INJURY, OR SEVERE PROPERTY OR ENVIRONMENTAL DAMAGE (“CRITICAL
APPLICATIONS”). TI SEMICONDUCTOR PRODUCTS ARE NOT DESIGNED, AUTHORIZED, OR
WARRANTED TO BE SUITABLE FOR USE IN LIFE-SUPPORT DEVICES OR SYSTEMS OR OTHER
CRITICAL APPLICATIONS. INCLUSION OF TI PRODUCTS IN SUCH APPLICATIONS IS UNDERSTOOD TO
BE FULLY AT THE CUSTOMER’S RISK.
In order to minimize risks associated with the customer’s applications, adequate design and operating
safeguards must be provided by the customer to minimize inherent or procedural hazards.
TI assumes no liability for applications assistance or customer product design. TI does not warrant or represent
that any license, either express or implied, is granted under any patent right, copyright, mask work right, or other
intellectual property right of TI covering or relating to any combination, machine, or process in which such
semiconductor products or services might be or are used. TI’s publication of information regarding any third
party’s products or services does not constitute TI’s approval, warranty or endorsement thereof.
Copyright  2000, Texas Instruments Incorporated
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