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High-Precision Real-Time Clock Modules
Features of Real-Time Clock Modules with built-in digital TCXO
Preface
Whether at work or play, the concept of time is a vital part of our daily lives. From systems that
dispatch trains to systems that record entry and exit times, innumerable applications depend on time data.
Timing has a huge influence on profit and loss in the world's financial and stock markets. With time and
clocks so essential to our everyday lives, clock functions have crept into countless everyday products in
recent years. In fact, it is said that finding a product that does not have a clock of some kind is difficult.
The world is also full of applications that require more accurate timekeeping, such as banking systems,
security systems, and electrical power meters, to name but a few. Essential in obtaining more accurate
clocks are (1) devices that oscillate at precise frequencies, and (2) ICs to control them. At Epson, we
manufacture and sell modules that combine into a single package a crystal unit that oscillates at a
precise, stable frequency and a real-time clock IC that controls the crystal. This article will explain the
features, functions, and characteristics of Epson's high-precision, low-power real-time clock modules.
Characteristics of Epson's Real-Time Clock Modules
A real-time clock module is a single package that contains a 32.768 kHz crystal unit and a real-time
clock IC, which includes oscillation circuitry clock, calendar, and alarm. At Epson, we develop and
manufacture our own crystal units and real-time clock ICs, giving us a stable supply of crystal units that
have been optimized for high-precision real-time clock modules and real-time clock ICs that operate
under the ideal conditions for those crystal units. Epson's semiconductor technology and expertise,
along with technology that enables extremely stable, low-power quartz oscillators, underlie watch
control. These technologies are found at the heart of myriad timing systems and timepieces, from the
official timekeeping systems used in the Olympics to luxury Seiko brand watches such as the Grand
Seiko.
Developing our own crystal units and real-time clock ICs enables us to design a perfect match and
bring out the full potential of both. This results in products that exhibit high performance. This is what
sets Epson's real-time clock modules apart from others.
The frequency accuracy of crystal units used in clock applications
Tuning-fork crystal units are typically used in
low-frequency clocks for timekeeping applications in
order to meet market requirements (for example, to
enable current times to be held at extremely low power
consumption).
While tuning-fork crystal units operate at low power, the
frequency-temperature coefficient exhibits a quadratic
curve, as seen in Fig. 1. Therefore, when designing the
clock error, it is necessary to account for not only the
frequency deviation at room temperature (+25 degrees
Celsius) but also the frequency-temperature coefficient
deviation of the quadratic curve.
If an ordinary tuning-fork crystal unit were to be operated
continuously for one month in a -40°C environment, the
oscillation frequency deviation would be -150 [x10
-6
], and the clock would lose 6 minutes/month or
more.
0
-10
-20
-30
-40
-50
-60
-70
-20 -10 0 10 20 30 40 50 60 70
Fig. 1. Tuning fork crystal frequency-temperature coefficient
Temperature (°C)
Frequency tolerance (×10
-6
)
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You might think, therefore, that you would rather use an AT-cut crystal or other crystal unit that has
a superior frequency-temperature coefficient as the frequency source. However, the oscillating
frequency of an AT-cut crystal unit is ordinarily on the order of several MHz, so the frequency would
have to be divided by an oscillation circuit to obtain the desired frequency for clock applications.
The electrical current consumed by the oscillation circuit at this time would be several hundred times
that consumed if a tuning-fork crystal unit were used. Therefore, we do not believe that using an AT-cut
crystal unit as a clock source meets market requirements.
Frequency accuracy compensation method using a digital TCXO
The oscillation frequency of tuning-fork crystal units changes along with changes in the ambient
temperature, as shown in Fig. 1 and requires a technique to compensate for these changes in order to
improve clock accuracy.
Epson uses digital TCXO temperature compensation to increase frequency accuracy. An outline of this
frequency accuracy compensation method is shown in Fig. 2.
In this method oscillation frequency is compensated by converting ambient temperature data to a
digital value at fixed frequencies and then retrieving from memory a compensation value that is suited
to that temperature. Methods for compensating oscillation frequency can broadly be divided into two
types: the load capacitance adjustment method and the clock update pulse adjustment method. The load
capacitance adjustment method is usually used in Epson's real-time clock modules.
These two compensation methods are explained below.
Load capacitance adjustment method
The load capacitance adjustment method corrects frequency by adjusting the crystal's oscillation
frequency. The oscillation frequency of a crystal unit can be changed by increasing or decreasing the
oscillation load capacitance. This method is used to correct variations in frequencies that occur in
response to changes in ambient temperature. This principle is schematically depicted in Fig. 3.
The frequency-temperature coefficient of a tuning-fork crystal unit is shown on the left side of Fig. 3.
The right side shows the load capacitance adjustment characteristics, wherein frequency changes
according to the load capacitance value. Specifically, the frequency drift (2) is calculated from the
ambient temperature data (1), and to derive the amount of change (3) in the load capacitance that
corresponds to that frequency drift. The amount of the change in load capacitance corresponding to that
temperature is retrieved as an offset value. The offset value is then applied to compensate the oscillation
frequency. Since this method compensates the oscillation frequency directly, the oscillation frequency
can be used as a low-frequency sleep clock whose oscillation output from the real-time clock module
has been compensated with high precision.
Capacitor array
Fig. 2 Schematic of frequency accuracy compensation
(load capacitance adjustment method)
Temp. sensor
ADC Controller
Compensation
data
Crystal unit
32.768 kHz
Oscillation
circuit