AN934
Application note
How to use the digital calibration feature in
TIMEKEEPER
®
and serial real-time clock (RTC) products
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 T
O
= 25 °C ± 5 °C and K = –0.036 ppm/°C
2
± 0.006 ppm/°C
2
.
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.
2.
It requires external components (trim capacitors); and
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
0
–35 ppm
–50
10
20
30
40
50
60
70
80
90
Temperature (°C)
–100
Minimum K at 25°C
Typical K at 25°C
Accuracy (ppm)
–150
Maximum K at 25°C
–200
AI02498
October 2011
Doc ID 6393 Rev 4
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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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Doc ID 6393 Rev 4
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
div 2
Calibration
Circuitry
64 Minute
Cycle
Clock
Registers
512Hz Output
for Frequency Test
256Hz
1Hz Signal
AI02800
Figure 3.
Clock splitting and clock blanking
No Calibration
Positive Clock Calibration
Negative Clock Calibration
AI02801
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Doc ID 6393 Rev 4
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 64-
minute 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 (C
L
). Thus, if the crystal has a C
L
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
y(2)
d5
S
Sign Bit
Calibration Value
d4
d3
d2
d1
d0
AI05651
1. x = W (Parallel device); OUT (Serial device)
2. y = R (Parallel device); FT (Serial device)
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