AN934
Application note
How to use the digital calibration feature in
and serial real-time clock (RTC) products
TIMEKEEPER®
Introduction
The term “quartz accurate” has become a familiar phrase used to describe the accuracy of
many time keeping functions. Quartz oscillators provide an accuracy far superior to other
conventional oscillator designs, but they are not perfect. Quartz crystals are sensitive to
temperature variations. Figure 1 shows the relationship between accuracy (acc),
temperature (T), and curvature (K) for the 32,768Hz crystal used on STMicroelectronics
TIMEKEEPER® products. The curves follow this general formula:
acc = K × ( T – T O )
2
where TO = 25 °C ± 5 °C and K = –0.036 ppm/°C2 ± 0.006 ppm/°C2.
The clocks used in most applications require a high degree of accuracy, and there are
several factors involved in achieving this accuracy. Typically most crystals are compensated
by adjusting the load capacitance of the oscillator. This method, while effective, has several
disadvantages:
1.
It requires external components (trim capacitors); and
2.
it can increase oscillator current (an important factor in battery-supported applications).
STMicroelectronics replaced this crude analog method with a digital calibration feature. This
method gives the user software control over the calibration procedure which makes it user
friendly.
Figure 1.
Typical crystal accuracy plotted against temperature (and against
different values of K)
50
+35 ppm
–40
–30
–20
10
–10
20
30
40
0
50
60
70
80
90
Temperature (°C)
–35 ppm
–50
–100
Minimum K at 25°C
–150
–200
Accuracy (ppm)
Typical K at 25°C
Maximum K at 25°C
AI02498
October 2011
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www.st.com
Contents
AN934
Contents
Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Calculating the needed amount of calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Calculating calibration over a temperature range . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Calculating the calibration for multiple operating temperatures . . . . . . . . . . . . . . 9
Enabling the frequency test function (FT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
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AN934
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Typical crystal accuracy plotted against temperature (and against different values of K) . . 1
Oscillator divider chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Clock splitting and clock blanking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Control register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Crystal accuracy over a temperature range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
A day of the week register (for parallel devices) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
512 Hz output to DQ0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
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Methodology
AN934
Methodology
The STMicroelectronics TIMEKEEPER® products are driven by a quartz crystal-controlled
oscillator with a nominal frequency of 32.768 kHz. The crystal is mounted in either a 600 mil
DIP CAPHAT™ package, 600 mil DIP hybrid, 300mil SOIC embedded crystal, or in a
330 mil SOIC SNAPHAT® package, along with the battery. A typical TIMEKEEPER device is
accurate within ±1.53 minutes (±35 ppm - parts per million) per month at 25 °C without
calibration.
Two sources of clock error are:
●
temperature variation
●
crystal variation
As mentioned previously, most clock chips compensate for crystal frequency and
temperature shift error with cumbersome “trim” capacitors. The TIMEKEEPER design
employs periodic counter correction. The digital calibration circuit adds or subtracts counts
from the oscillator divider circuit at the 256 Hz stage (see Figure 2).
Figure 3 shows how extra clock pulses are added (by clock splitting) or removed (by clock
blanking). The number of times the pulses are split (added during positive calibration) or
blanked (subtracted during negative calibration) depends upon the value that has been
loaded into the least significant five bits of the control register. Adding counts speeds the
clock up while subtracting counts slows the clock down.
Figure 2.
Oscillator divider chain
32768Hz
Low
Current
Oscillator
div 64
512Hz Output
for Frequency Test
div 2
Calibration
Circuitry
64 Minute
Cycle
256Hz
1Hz Signal
Figure 3.
Clock
Registers
AI02800
Clock splitting and clock blanking
No Calibration
Positive Clock Calibration
Negative Clock Calibration
AI02801
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AN934
Methodology
The calibration byte occupies the five lower order bits in the control register, as shown in
Figure 4. These bits represent the binary value between 0 and 31. Table 1 on page 6 shows
how many seconds (or ppm) each bit represents in real time for the TIMEKEEPER® product
line. The sixth bit is a sign bit. A binary '1' indicates a positive calibration (added pulses), and
a binary '0' indicates a negative calibration (blanked pulses). Calibration occurs within a 64minute cycle. The first 62 minutes in the cycle may, once per minute, have one second either
shortened by 128 or lengthened by 256 oscillator cycles. If a binary '1' is loaded into the
register, only the first 2 minutes in the 64-minute cycle are modified; if a binary “6” is loaded,
the first 12 minutes are affected, and so on.
Therefore, each calibration step has the effect of adding 512 or subtracting 256 oscillator
cycles for every 125,829,120 actual oscillator cycles (64 minutes x 60 seconds/minute x
32,768 cycles/second). That is, +4.068 or –2.034 ppm of adjustment per calibration step in
the calibration register. Assuming that the oscillator is running exactly at 32.768kHz, each of
the 31 increments in the calibration byte represent +10.7 or –5.35 seconds per month, which
corresponds to a total range of +5.5 or –2.75 minutes per month.
As can be seen from Figure 1 on page 1, the peak of the curve corresponds to
approximately 25 °C. This is known as the “turnover temperature.” As the temperature rises
or falls from room temperature, the oscillator slows down. Typically the turnover point on the
graph is very close to 32.768 kHz (no error). However, variations from one crystal to another
may cause the turnover point to be slightly above or below 32.768 kHz. The frequency
variation for an uncalibrated device is a function of the crystal frequency variation for the
given load capacitance (CL). Thus, if the crystal has a CL that is different from the actual
internal load capacitance of the device, then the oscillator frequency will run faster or slower
than the 32.768 kHz (±1 Hz). At STMicroelectronics, the real-time clock has an internal
capacitance of 12.5 pF (except for the M41T6x series, which has an internal capacitance of
6 pF) across the crystal input pins. For this reason, the calibration feature can be
programmed to adjust for both negative and positive variations. Entering a value into the
6-bit calibration field of the control register will shift the entire curve up or down according to
the values found in Table 1 on page 6.
Figure 4.
d7
x(1)
Control register
d6
d5
y(2)
S
d4
d3
d2
d1
d0
Calibration Value
Sign Bit
AI05651
1. x = W (Parallel device); OUT (Serial device)
2. y = R (Parallel device); FT (Serial device)
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Methodology
Table 1.
AN934
Calibration table: compensation values in seconds per month (30 days) and in ppm
Calibration
value (binary)
Value in seconds per month (30 days)
rounded to the nearest second
Value in ppm
rounded to the nearest ppm
Plus
Minus
Plus
Minus
00000
0
0
0
0
00001
11
5
4
2
00010
21
11
8
4
00011
32
16
12
6
00100
42
21
16
8
00101
53
26
20
10
00110
63
32
24
12
00111
74
37
28
14
01000
84
42
33
16
01001
95
47
37
18
01010
105
53
41
20
01011
116
58
45
22
01100
127
63
49
24
01101
137
69
53
26
01110
148
74
57
29
01111
158
79
61
31
10000
169
84
65
33
10001
179
90
69
35
10010
190
95
73
37
10011
200
100
77
39
10100
211
105
81
41
10101
221
111
85
43
10110
232
116
89
45
10111
243
121
93
47
11000
253
127
98
49
11001
264
132
102
51
11010
274
137
106
53
11011
285
142
110
55
11100
295
148
114
57
11101
306
153
118
59
11110
316
158
122
61
11111
327
163
126
63
337*N/32
169*N/32
337*N/(32*2.592)
169*N/(32*2.592)
In general:
N
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AN934
Calculating the needed amount of calibration
Calculating the needed amount of calibration
There are two methods for establishing how much calibration are required in a given
application.
The first method can be easily implemented in the user environment simply by setting the
clock to a known accurate reference and then storing the time in some unused location in
the RAM. Over a period of time (30 days), the reference time is compared to the current time
(the average ambient temperature should be considered as well). This will tell the user how
fast or slow the clock operates for a period of 30 days. While it may seem crude, it allows the
designer to give the end user the ability to calibrate the clock according to the specified
environment. The ability to calibrate the clock can also be accomplished even after the final
product is packaged in a non-user serviceable enclosure by providing a simple utility to
access the calibration byte.
Table 1 on page 6 provides a direct look-up table for calibration values based upon the
number of seconds lost or gained during a one month (30 day) period. For example, if the
system were to lose 20 seconds during one month, the needed calibration would be +20
seconds. The user could look up a +20 second value in the table under the appropriate
column. In this case, the nearest value is +21. The appropriate sign bit in this case is a
logical '1,' indicating the clock needs to speed up to compensate for the lost time. This yields
a calibration value of “100010.” In this example, the inaccuracy would be reduced from –20
seconds per month to +1 second per month.
The second approach is better suited for a manufacturing environment, and involves the use
of a special test mode (as described in the section entitled, “Enabling the frequency test
function (FT)” which derives a 512Hz signal from the clock divider chain, as indicated in
Figure 2. This signal can be used to measure the accuracy of the crystal oscillator. The
right-hand pair of columns in Table 1 on page 6 provides a look-up table similar to that in the
left-hand pair of columns, except that the error values are expressed in “ppm” units instead
of seconds per month. The error in ppm can be quickly calculated by dividing the measured
error from 512 Hz by 512 and multiplying the result by 1 million. For example, if the
frequency measured during the test mode is 511.998 Hz, the delta is –0.002. Dividing by
512 and multiplying by 1 million, the result is –3.906 ppm. In this case, the nearest
compensation value is a +4.068. The appropriate sign bit in this case is a logical '1,'
indicating the clock needs to speed up to compensate for the lost time. This yields a
calibration value of “100001.”
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Calculating calibration over a temperature range
AN934
Calculating calibration over a temperature range
The calibration procedure described so far has centered around calculating the correction
for a specific temperature. This section considers the procedure for minimizing the
frequency error over a wider temperature range. This involves adjusting the frequency curve
so that there is an equal amount of error above and below the zero (0) ppm point. Figure 5
shows how the frequency error can be minimized over a given temperature range.
The variables in the equation (see Introduction on page 1) are as follows:
K = Curvature characteristic = –0.036 ppm/°C2 ± 0.006 ppm/°C2
acc = Accuracy, in ppm, of the frequency, at the turnover temperature
TO = Turnover temperature, in degrees Celsius = 25 °C ± 5 °C
T = Working temperature, in degrees Celsius
For example, if a device is in error by +20 ppm at room temperature, but will actually operate
at –20 °C in the application, the equation on page 1 may be used to calculate the required
calibration value as follows:
acc = 20 ppm + (–0.036ppm/°C2) * (–20°C – 25°C)2
acc = –52.9 ppm
Since the unit will be slow by 52.9 ppm, the required correction is +52.9 ppm, and this can
be looked up in Table 1 on page 6; the nearest value is a +53. The appropriate sign bit in
this case is a logical '1,' indicating the clock needs to speed up to compensate for the lost
time. This yields a calibration value of “101101.”
Figure 5.
Crystal accuracy over a temperature range
50
20
–40
–30
–20
10
–10
20
30
40
50
60
70
0
80
90
Temperature °C
–50
After calibration
Before calibration
–150
–200
8/14
Accuracy (ppm)
–100
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AI02499
AN934
Calculating the calibration for multiple operating temperatures
Calculating the calibration for multiple operating
temperatures
For applications that spend significant time at more than one temperature, the following
equation may be used to calculate the appropriate amount of calibration required:
N
t =
2
t PERi ( acc + K × ( T i – T o ) ) × 10
Σ
–6
i=1
where:
K = Curvature characteristic = –0.036 ppm/°C2 ± 0.006 ppm/°C2
acc = Accuracy, in ppm, of the frequency, at the turnover temperature
TO = Turnover temperature, in degrees Celsius = 25°C ± 5°C
Ti = Working temperature, in degrees Celsius
tPERi = Amount of time it is in the temperature range (in seconds)
t = Amount of time lost during tPERi
N = Number of temperature ranges
Consider a piece of portable equipment used outdoors for 8 hours per day, then stored at
room temperature for the remainder of the day. The equation below calculates the
calibration value at –20°C for a period of 8 hours and then room temperature for the rest of
the day for a device that is currently in error by +5 ppm at room temperature:
8hours = 28800 sec onds
16hours = 57600 sec onds
2
2
= { [ ( 28800 sec s ) × ( 5 + ( ( – 0.036 ppm ⁄ ° C ) × ( – 20°C – 25°C ) ) ) ] +
[ ( 57600 sec s ) × ( 5 + 0 ) ] } × 10
–6
t = – 1.67 sec s ⁄ day
The unit is losing 1.67 seconds per day (or 50 seconds per month). The appropriate sign bit
in this case is a logical “1,” indicating the clock needs to be sped up to compensate for the
lost time. This yields a calibration value of “100101.”
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Enabling the frequency test function (FT)
AN934
Enabling the frequency test function (FT)
Figure 4 and 6 show the location of the Frequency Test (FT) bit, DQ6, of the day-of-theweek register for the parallel device or DQ6 in the control register for the serial device.
Setting the FT bit to a '1' turns on the frequency test mode. The user needs to make sure
that the Stop (ST) bit in the second register, is set to a '0.' Exactly where the 512 Hz signal is
output depends on which TIMEKEEPER® device is being used, as indicated in Table 2. On
the M48T02, M48T12, M48T08, M48T18, and M48T35 devices, the 512 Hz signal is output
on the DQ0 pin when the device is reading the Seconds register. The address and control
signals must be valid during the measurement process, as shown in Figure 7 on page 10.
On the M41T62, M41T63, and M41T64, the 512 Hz signal is output on the SQW pin. To
enable the 512 Hz signal, the SQWE bit = 1 (DQ6 of the alarm month register), RS3 = 0,
RS2 = 1, and RS1 = 0 (DQ7-DQ4 in the day register). The SQW output pin is an open drain
output for M41T64 and a full CMOS output for the M41T62 and M41T63.
For all other devices listed in Table 2, the 512 Hz signal is output on the FT/OUT, FT,
IRQ/FT, and IRQ/FT/OUT pins. These outputs are open drain, and require a pull-up resistor.
A 500 Ω to 10 kΩ resistor is recommended in order to control the rise time. Measurement
should be taken from negative edge to negative edge due to the slow rise time on the
positive edge. If the IRQ function is enabled, the FT function is inhibited.
Note:
Setting or changing the calibration byte does not affect the frequency test output frequency
as the adjustment is made at the 256 Hz stage.
Once the amount of calibration has been determined, either from the test mode or by
monitoring it over a period of time, the user can enter the values from the calibration tables
into the control register.
Figure 6.
A day of the week register (for parallel devices)
d7
d6
d4
d5
d3
d2
d1
d0
FT
Freq.
Test Bit
Day of the Week
AI05652
Figure 7.
512 Hz output to DQ0
Address Bus
Address for Seconds Register
E
G
DQ0
512Hz Output
AI02802
Note:
10/14
Care should be taken when writing to the control register so as not to overwrite the
calibration value.
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AN934
Enabling the frequency test function (FT)
Table 2.
512 Hz output pin
Device
M41T00, M41T00S, M41T11, M41T56, M41T00CAP
M48T02, M48T12, M48T08, M48T08Y, M48T18, M48T35, M48T35AV, M48T35Y
M41T60, M48T58, M48T58Y
Pin name
FT/Out
DQ0
FT
M48T37V, M48T37Y, M48T201V, M48T201Y
M41ST85W, M41ST87W, M41ST87Y, M41ST95W, M41T65, M41T81, M41T81S,
M41T94, M41T00AUD, M41T93
M41T62, M41T63, M41T64, M41T66
IRQ/FT
IRQ/FT/OUT
SQW
M41T82
FT/RST
M41T83
IRQ1/OUT/FT
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Conclusion
AN934
Conclusion
Software calibration is a convenient feature which allows the user to adjust the clock
accuracy during manufacturing (or later) at minimal cost. This feature also provides a
method whereby “drift” (due to temperature variation) can be corrected and/or anticipated.
See http://www.st.com for additional details as well as an online calibration calculation
tool.
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AN934
Revision history
Revision history
Table 3.
Document revision history
Date
Revision
Changes
Dec-1998
1
20-May-2003
1.1
16-Feb-2004
2
Update web reference information
11-Nov-2004
3
Reformatted; updates to content (Figure 4, 6;Table 2)
07-Oct-2011
4
Updated products, title of application note, Table 2; revised document
presentation.
First edition
Clarify compensation required (Table 1, 2); add Conclusion
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AN934
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