2013 Microchip Technology Inc. DS00001519B-page 1 AN1519 This document is designed to serve as a starting point when choosing a crystal to operate alongside the Microchip Stand-Alone Real-Time Clock/Calendar devices (Figure 1). FIGURE 1: OSCILLATOR DIAGRAM To oscillate as closely as possible to the desired fre- quency, a crystal must have load capacitors that match the value recommended by the crystal manufacturer, according to Equation 1. Also, the oscillator pin capacitance (available in the device data sheet as COSC) must be included in CX1 and CX2, and stray board capacitance (C stray ) must be taken into consideration when choosing the capacitors. EQUATION 1: CONSIDERATIONS The Microchip stand-alone RTCC’s have been designed to work with 32.768 kHz tuning fork crystals with a load capacitance (CLOAD or CL) of 6-9pF. For tuning fork crystals, the frequency has a parabolic dependence on temperature. Therefore, when it changes, the frequency decreases accordingly, as shown in Equation 2 and Figure 2. See AN1413, “Tem- perature Compensation of a Tuning Fork Crystal Based on MCP79410” (DS01413). EQUATION 2: FIGURE 2: PARABOLIC CURVE FOR TUNING FORK CRYSTALS Author: Florian Gheorghe Microchip Technology Inc. X1 X2 RTCC C X1 C X2 C load C x2 C x1 C x2 C x1 + ------------------------ +C stray = Where: C x1 = Capacitor value on pin X1 + C pin C x2 = Capacitor value on pin X2 + C pin C stray = Trace capacitance C pin = 3 pF typical f f 0 [1-Tc T T 0 – 2 = Where: f 0 = Frequency at turnover point Tc = Temperature coefficient T-T 0 = Deviation from turnover point T = Current temperature (°C) T 0 = Turnover point (°C) Recommended Crystals for Microchip Stand-Alone Real-Time Clock/Calendar Devices
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AN1519Recommended Crystals for Microchip
Stand-Alone Real-Time Clock/Calendar Devices
This document is designed to serve as a starting pointwhen choosing a crystal to operate alongside theMicrochip Stand-Alone Real-Time Clock/Calendardevices (Figure 1).
FIGURE 1: OSCILLATOR DIAGRAM
To oscillate as closely as possible to the desired fre-quency, a crystal must have load capacitors that matchthe value recommended by the crystal manufacturer,according to Equation 1.
Also, the oscillator pin capacitance (available in thedevice data sheet as COSC) must be included in CX1and CX2, and stray board capacitance (Cstray) must betaken into consideration when choosing the capacitors.
EQUATION 1:
CONSIDERATIONSThe Microchip stand-alone RTCC’s have beendesigned to work with 32.768 kHz tuning fork crystalswith a load capacitance (CLOAD or CL) of 6-9pF. Fortuning fork crystals, the frequency has a parabolicdependence on temperature. Therefore, when itchanges, the frequency decreases accordingly, asshown in Equation 2 and Figure 2. See AN1413, “Tem-perature Compensation of a Tuning Fork Crystal Basedon MCP79410” (DS01413).
EQUATION 2:
FIGURE 2: PARABOLIC CURVE FOR TUNING FORK CRYSTALS
Where:Cx1 = Capacitor value on pin X1 + CpinCx2 = Capacitor value on pin X2 + CpinCstray = Trace capacitanceCpin = 3 pF typical
f f0 [1-Tc T T0– 2 =
Where: f0 = Frequency at turnover pointTc = Temperature coefficientT-T0 = Deviation from turnover pointT = Current temperature (°C)T0 = Turnover point (°C)
2013 Microchip Technology Inc. DS00001519B-page 1
AN1519
For best results, it is recommended that a ground ringshould encompass the crystal and the X1 and X2 pins.See AN1365, “Recommended Usage of MicrochipSerial RTCC Devices” (DS01365). Also, the tracesfrom the RTCC to the capacitors and crystal should beas short as possible in order to minimize the strayboard capacitance (CSTRAY). See AN1288, “DesignPractices for Low-Power External Oscillators”(DS01288).
Table 2 shows recommended crystals and loadcapacitors.
Some crystal vendors use the term oscillation allow-ance as the sum of negative R value and ESR(Equation 3). The negative R (-R) which has been mea-
sured on the AC164140 RTCC PICtail™ board is ameasure of the ability of the oscillator to drive the crys-tal over temperature (Figure 3). An oscillation allow-ance value of three to five times the crystal ESR willprovide an acceptable margin. See AN943, “PracticalPICmicro® Oscillator Analysis and Design” (DS00943)and AN949, “Making Your Oscillator Work” (DS00949).
CRYSTAL TEST RESULTSThe crystals listed in Table 1 have been tested on theAC164140 RTCC PICtail board (unless noted). Theresults are in Table 2.
FIGURE 12: FREQUENCY VOLTAGE CHARACTERISTIC FOR C1 = 8.2 PF, C2 = 8.2 PF
I.2 HK 2T 6PF
FIGURE 13: FREQUENCY VOLTAGE CHARACTERISTIC FOR C1 = 8.2 PF, C2 = 10 PF
32,767.850
32,767.852
32,767.854
32,767.856
32,767.858
32,767.860
32,767.862
32,767.864
32,767.866
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Freq
uenc
y (H
z)
Voltage (V)
C1 = 8.2 pF C2 = 8.2 pF
32,768.042
32,768.044
32,768.046
32,768.048
32,768.050
32,768.052
32,768.054
32,768.056
32,768.058
32,768.060
32,768.062
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Freq
uenc
y (H
z)
Voltage (V)
C1 = 8.2 pF C2 = 10 pF
DS00001519B-page 8 2013 Microchip Technology Inc.
AN1519
I.3 HK M3 6PF
FIGURE 14: FREQUENCY VOLTAGE CHARACTERISTIC FOR C1 = 8.2 PF, C2 = 10 PF
I.4 HK M8 7PF
FIGURE 15: FREQUENCY VOLTAGE CHARACTERISTIC FOR C1 = 10 PF, C2 = 12 PF
32,768.1040
32,768.1045
32,768.1050
32,768.1055
32,768.1060
32,768.1065
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Freq
uenc
y (H
z)
Voltage (V)
C1 = 8.2 pF C2 = 10 pF
32,767.74
32,767.76
32,767.78
32,767.80
32,767.82
32,767.84
32,767.86
32,767.88
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Freq
uenc
y (H
z)
Voltage (V)
C1 = 10 pF C2 = 12 pF
2013 Microchip Technology Inc. DS00001519B-page 9
AN1519
I.5 HK M8 9PF
FIGURE 16: FREQUENCY VOLTAGE CHARACTERISTIC FOR C1 = 15 PF, C2 = 12 PF
I.6 HK M3 8PF
FIGURE 17: FREQUENCY VOLTAGE CHARACTERISTIC FOR C1 = 12 PF, C2 = 12 PF
32,768.02
32,768.04
32,768.06
32,768.08
32,768.10
32,768.12
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Freq
uenc
y (H
z)
Voltage (V)
C1 = 15 pF C2 = 12 pF
32,767.920
32,767.925
32,767.930
32,767.935
32,767.940
32,767.945
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Freq
uenc
y (H
z)
Voltage (V)
C1 = 12 pF C2 = 12 pF
DS00001519B-page 10 2013 Microchip Technology Inc.
AN1519
I.7 HK 2T 8PF
FIGURE 18: FREQUENCY VOLTAGE CHARACTERISTIC FOR C1 = 12 PF, C2 = 12 PF
I.8 HK 3T 8PF
FIGURE 19: FREQUENCY VOLTAGE CHARACTERISTIC FOR C1 = 12 PF, C2 = 15 PF
32,768.110
32,768.115
32,768.120
32,768.125
32,768.130
32,768.135
32,768.140
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Freq
uenc
y (H
z)
Voltage (V)
C1 = 12 pF C2 = 12 pF
32,768.142
32,768.144
32,768.146
32,768.148
32,768.150
32,768.152
32,768.154
32,768.156
32,768.158
32,768.160
32,768.162
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Freq
uenc
y (H
z)
Voltage (V)
C1 = 12 pF 2 = 15 pF
2013 Microchip Technology Inc. DS00001519B-page 11
AN1519
NOTES:
DS00001519B-page 12 2013 Microchip Technology Inc.
Note the following details of the code protection feature on Microchip devices:• Microchip products meet the specification contained in their particular Microchip Data Sheet.
• Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the intended manner and under normal conditions.
• There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data Sheets. Most likely, the person doing so is engaged in theft of intellectual property.
• Microchip is willing to work with the customer who is concerned about the integrity of their code.
• Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not mean that we are guaranteeing the product as “unbreakable.”
Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of ourproducts. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such actsallow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act.
Information contained in this publication regarding deviceapplications and the like is provided only for your convenienceand may be superseded by updates. It is your responsibility toensure that your application meets with your specifications.MICROCHIP MAKES NO REPRESENTATIONS ORWARRANTIES OF ANY KIND WHETHER EXPRESS ORIMPLIED, WRITTEN OR ORAL, STATUTORY OROTHERWISE, RELATED TO THE INFORMATION,INCLUDING BUT NOT LIMITED TO ITS CONDITION,QUALITY, PERFORMANCE, MERCHANTABILITY ORFITNESS FOR PURPOSE. Microchip disclaims all liabilityarising from this information and its use. Use of Microchipdevices in life support and/or safety applications is entirely atthe buyer’s risk, and the buyer agrees to defend, indemnify andhold harmless Microchip from any and all damages, claims,suits, or expenses resulting from such use. No licenses areconveyed, implicitly or otherwise, under any Microchipintellectual property rights.
2013 Microchip Technology Inc.
QUALITY MANAGEMENT SYSTEM CERTIFIED BY DNV
== ISO/TS 16949 ==
Trademarks
The Microchip name and logo, the Microchip logo, dsPIC, FlashFlex, KEELOQ, KEELOQ logo, MPLAB, PIC, PICmicro, PICSTART, PIC32 logo, rfPIC, SST, SST Logo, SuperFlash and UNI/O are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries.
FilterLab, Hampshire, HI-TECH C, Linear Active Thermistor, MTP, SEEVAL and The Embedded Control Solutions Company are registered trademarks of Microchip Technology Incorporated in the U.S.A.
Silicon Storage Technology is a registered trademark of Microchip Technology Inc. in other countries.
Analog-for-the-Digital Age, Application Maestro, BodyCom, chipKIT, chipKIT logo, CodeGuard, dsPICDEM, dsPICDEM.net, dsPICworks, dsSPEAK, ECAN, ECONOMONITOR, FanSense, HI-TIDE, In-Circuit Serial Programming, ICSP, Mindi, MiWi, MPASM, MPF, MPLAB Certified logo, MPLIB, MPLINK, mTouch, Omniscient Code Generation, PICC, PICC-18, PICDEM, PICDEM.net, PICkit, PICtail, REAL ICE, rfLAB, Select Mode, SQI, Serial Quad I/O, Total Endurance, TSHARC, UniWinDriver, WiperLock, ZENA and Z-Scale are trademarks of Microchip Technology Incorporated in the U.S.A. and other countries.
SQTP is a service mark of Microchip Technology Incorporated in the U.S.A.
GestIC and ULPP are registered trademarks of Microchip Technology Germany II GmbH & Co. KG, a subsidiary of Microchip Technology Inc., in other countries.
All other trademarks mentioned herein are property of their respective companies.
Microchip received ISO/TS-16949:2009 certification for its worldwide
DS00001519B-page 13
headquarters, design and wafer fabrication facilities in Chandler and Tempe, Arizona; Gresham, Oregon and design centers in California and India. The Company’s quality system processes and procedures are for its PIC® MCUs and dsPIC® DSCs, KEELOQ® code hopping devices, Serial EEPROMs, microperipherals, nonvolatile memory and analog products. In addition, Microchip’s quality system for the design and manufacture of development systems is ISO 9001:2000 certified.
DS00001519B-page 14 2013 Microchip Technology Inc.
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