Considerations for Maxim Real-Time Clock Crystal Selection
Abstract: This application note describes crystal selection criteria and proper layout techniques for connecting a 32kHz crystal to a real-time clock (RTC). Following these guidelines will prevent time errors and incorrect time due to crosstalk and improper crystal selection.
All Maxim real-time-clock (RTC) products require an external 32.768kHz watch crystal for use on their on-board crystal oscillator circuit. Except for this crystal, no other external components are required. Selection of the external crystal varies depending on the end application. Considerations are operating temperature range, package style/size, crystal load capacitance, and cost.
Crystal Manufacturers
There are many crystal manufacturers for 32.768kHz "watch crystals." A few of them are listed below, along with their web addresses for quick access to technical details regarding crystals.
Manufacturer
Website
Raltron
http://www.raltron.com
ECS INC International
http://www.ecsxtal.com
SaRonix
http://www.saronix.com
Abracon Corporation
http://www.abracon.com
Fox Electronics
http://www.foxonline.com
M-tron
http://www.mtron.com
Caliber Electronics
http://www.caliberelectronincs.com
Common Watch-Crystal Specifications
Watch crystals at 32.768kHz are optimized for either 6pF or 12.5pF loads (refer to specific Maxim RTC data sheets for which crystal is required). There are common electrical characteristics for both, and these specifications are listed below. In addition, there is a common parabolic temperature curve for watch crystals, shown in Figure 1. Above or below +25°C, the frequency becomes lower with changes in temperature as determined by the frequency stability constant K, which has a maximum value of -0.040 PPM/(Δ°C)².
Parameter
Symbol
Min
Typ
Max
Units
Frequency
f
32.768
kHz
Equivalent Series Resistance (ESR)
Rs
35
60
KΩ
Frequency Stability
K
-0.028
-0.040
PPM/(Δ°C)²
Turnover Temperature
To
+20
+30
°C
Drive Level
1.0
µW
Aging (first year @ 25 °C)
-3.0
+3.0
PPM
Load Capacitance
6pF or 12.5pF
Q Factor
Q
40,000
60,000
Figure 1. Parabolic temperature curve.
In general, an RTC that uses a 12.5pF crystal will have a timekeeping current of 1.7X more than an RTC that uses a 6pF crystal (that is, 6pF RTC current = 300nA @2V whereas 12.5pF RTC current = 500nA@2V). Timekeeping current is that measured when there is no serial bus activity and the RTC is only using current to run its 32.768kHz oscillator and count real time. A standard, low-cost, 50ma-hr, lithium, backup battery powers an RTC with timekeeping current of less than 570nA for up to 10 years.
A 12.5pF load crystal oscillator is somewhat more stable and less susceptible to noise and PCB layout stray capacitance than a 6pF load crystal oscillator. This is partly due to the capacitance from each crystal pin to ground, internal to the RTC, which is 25pF per pin for the 12.5pF crystal RTC and 12pF for the 6pF crystal RTC.
There are also inventory issues for 32.768kHz watch crystals. Load crystals at 12.5pF are readily available through many distributors. Load crystals at 6pF are not as readily available and can require a minimum-quantity order to procure. Check with your local supplier for availability and price.
It is important to use the correct specified crystal with an RTC, for the wrong crystal can cause as much as a 100ppm error in the 32.768kHz oscillator frequency, which translates to a 4.3-minute error over a one-month period. In addition, the wrong crystal can cause excessive timekeeping current or failure of the oscillator to start properly.
Through-Hole Leaded Crystals
There are three common packages for through-hole leaded crystals. They are available in both 6pF and 12.5pF crystal loads. These package styles are shown below, along with manufacturers who offer such packages.
Figure 2. Package for through-hole leaded crystals.
REF
TH1
TH2
TH3
A
5.0 max
6.0 max
8.3 max
B
1.5 max
2.0 max
3.2 max
C
4.3 min
4.0 min
9.0 min
D
0.17 +/-0.05
0.2 +/-0.15
0.3 +/-0.07
E
0.45
0.7 +/-0.15
1.1 +/-0.2
Note: Dimensions in mm
Manufacturer
Crystal Load
Operating Temperature Range (°C)
Frequency Tolerance @ 25°C
Part Series
Package Style
Raltron
12.5pF optional; 6pF optional
-10 to +60
+/-20PPM
R145
TH1
ECS INC International
12.5pF optional; 6pF optional
-10 to +60
+/-20PPM
ECS-1X5
TH1
SaRonix
TH1
Abracon Corporation
TH1
Fox Electronics
12.5pF
-20 to +60
+/-20PPM
NC15
TH1
M-tron
12.5pF optional; 6pF optional
-20 to +70
+/-30PPM +/-20PPM
MMCC-3
TH1
Caliber Electronics
12.5pF
-10 to +60
+/-20PPM
AWT3
TH1
Manufacturer
Crystal Load
Operating
Temperature Range (°C)
Frequency
Tolerance @ 25°C
Part
Series
Package
Style
Raltron
12.5pF standard; 6pF optional
-10 to +60
+/-20PPM
R26
TH2
ECS INC International
12.5pF
-40 to +85
+/-20PPM
ECS-2X6-FL
TH2
SaRonix
6pF optional; 12.5pF optional
-10 to +60
+/-20PPM
NTF3226
TH2
Abracon Corporation
12.5pF standard; 6pF optional
-10 to +60 -40 to +85 optional
+/-20PPM +/-10PPM +/-5PPM
AB26T
TH2
Fox Electronics
12.5pF
-20 to +60
+/-20PPM
NC26
TH2
M-tron
12.5pF
-20 to +70
+/-30PPM +/-20PPM
MMCC-2
TH2
Caliber Electronics
12.5pF
-10 to +60
+/-20PPM
AWT2
TH2
Manufacturer
Crystal
Load
Operating
Temperature Range (°C)
Frequency
Tolerance @ 25°C
Part
Series
Package
Style
Raltron
12.5pF standard; 6pF optional
-10 to +60
+/-20PPM
R38
TH3
ECS INC International
TH3
SaRonix
12.5pF
-10 to +60
+/-20PPM
NTF3238
TH3
Abracon Corporation
12.5pF standard; 6pF optional
-10 to +60 -40 to +85 optional
+/-20PPM +/-10PPM +/-5PPM
AB38T
TH3
Fox Electronics
12.5pF
-20 to +60
+/-20PPM
NC38
TH3
M-tron
12.5pF
-20 to +70
+/-30PPM +/-20PPM
MMCC-1
TH3
Caliber Electronics
12.5pF
-10 to +60
+/-20PPM
AWT
TH3
Surface-Mount Crystals
There are three common packages for surface-mount crystals. They are available in both 6pF and 12.5pF crystal loads. These package styles are shown below, along with manufacturers who offer such packages.
Figure 3. Surface-mount crystals (all dimensions are in mm).
Manufacturer
Crystal
Load
Operating
Temperature Range (°C)
Frequency
Tolerance @ 25°C
Part
Series
Package
Style
Raltron
12.5pF
-10 to +60
-10 to +60
RSM200SA RSM200SB
SM1A SM1B
Raltron
6pF 12.5pF
-40 to +85
+/-20PPM +/-50PPM +/-100PPM
RSE B RSE A
SM1B SM1A
ECS INC International
12.5pF standard 6pF optional
-40 to +85
+/-20PPM
ECX-205 ECX-206
SM1B SM1A
SaRonix
12.5pF
-40 to +85
+/-20PPM
32S12B 3212SA
SM1B SM1A
Abracon Corporation
12.5pF standard 6pF optional
-40 to +85
+/-20PPM +/-25PPM +/-30PPM
ABS35A ABS35B
SM1B SM1A
Fox Electronics
12.5pF
-40 to +85
+/-20PPM
FSM327
SM1B
M-tron
12.5pF
-20 to +70
+/-20PPM
SSX2010 SSX2020
SM1B SM1A
Caliber Electronics
12.5pF
-20 to +70
+/-20PPM
AWS2A AWS2B
SM1B SM1A
Manufacturer
Crystal
Load
Operating
Temperature Range (°C)
Frequency
Tolerance @ 25°C
Part
Series
Package
Style
Raltron
6pF 12.5pF
-40 to +85
+/-20PPM +/-50PPM +/-100PPM
RSE H2
SM2
ECS INC International
6pF 12.5pF
-40 to +85
+/-20PPM
ECX-3TA
SM2
SaRonix
SM2
Abracon Corporation
12.5pF standard 6pF optional
-40 to +85
+/-20PPM +/-25PPM +/-30PPM
ABS20G
SM2
Fox Electronics
12.5pF
-40 to +85
+/-20PPM
FSP
SM2
Manufacturer
Crystal
Load
Operating
Temperature Range (°C)
Frequency
Tolerance @ 25°C
Part
Series
Package
Style
Raltron
12.5pF
-10 to +60
+/-20PPM
RSM200S
SM3A
Raltron
6pF 12.5pF
-40 to +85
+/-20PPM +/-50PPM +/-100PPM
RSE C RSE D
SM3A
SM3B
ECS INC International
12.5pF standard 6pF optional
-40 to +85
+/-20PPM
ECX-306I
SM3A
SaRonix
12.5pF
-40 to +85
+/-20PPM
32S12C
SM3A
Abracon Corporation
12.5pF standard 6pF optional
-40 to +85
+/-30PPM +/-20PPM +/-25PPM
ABS25
SM3A
Fox Electronics
12.5pF
-40 to +85
+/-20PPM
FSR327 FST327
SM3A
SM3B
M-tron (SM3 pkg w/2mm height)
12.5pF
-40 to +85
+/-30PPM
SX1575
SM3A
M-tron
12.5pF
-20 to +70
+/-30PPM
SX1555
SM3A
Typical Crystal Prices
Typical distributor pricing (as of early 2001) in 1K quantity is shown below. Regional pricing and availability can vary and should be checked with your local supplier.
Package Style
Price in 1K Qty*
TH1
$0.62
TH2
$0.18
TH3
$0.18
SM1
$0.62
SM2
$0.82
SM3
$0.62
*Catalog distributor pricing in $US
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