Characteristics of TCXO RTC Real time Clock Module

[Preface]

Time is a fundamental concept in our daily lives. Various processing systems and applications for train operation management, entry and exit management, etc. work according to time information. In the fields of finance and stock markets, time manipulates huge profits and losses. Time is an indispensable part of our lives. In recent years, products around us have also been equipped with time functions, making it difficult to find products without time functions. There are still numerous applications in various fields of society that require more precise timing, such as financial processing systems, security systems, electricity meters, etc. In order to obtain more accurate time, it is necessary to have components with high-precision oscillation frequency and chips for controlling components. The modules produced and sold by Epson integrate high-precision and high stable frequency quartz crystal oscillators with real-time clock chips for control functions. This time, we will explain the features (functions) and structure of Epson's high-precision, low-power real-time clock module.

【Features of Epson Real Time Clock Module】

The real-time clock module is a product that integrates a 32.768kHz quartz crystal oscillator and a real-time clock chip, with oscillation circuit, clock function, calendar function, and alarm function. Epson independently develops and produces quartz crystal oscillators and real-time clock chips used in real-time clock modules. Therefore, it is possible to stably supply the quartz crystal oscillator that is most suitable for high-precision real-time clock modules, as well as the real-time clock chip that drives the oscillator under optimal conditions. Moreover, the application of Epson semiconductor technology started with the world's first practical quartz electronic watch, and has also been used for the Olympic formula timing system and the heart control of Seiko high-end watches represented by "Grand Seiko". These semiconductor technologies used for controlling chips, combined with outstanding low-power, high stability quartz oscillation technology, form the foundation of high-quality real-time clock modules.

As mentioned above, we have developed our own quartz crystal oscillator and real-time clock chip to achieve optimal matching and maximize the strengths of both parties, thereby providing customers with high-performance products. This is precisely the characteristic of Epson's real-time clock module.

【Frequency accuracy of quartz crystal oscillator used for clocks】

According to market requirements (able to maintain the current time with extremely low power consumption), low-frequency clocks used for timing generally use tuning fork type quartz crystal oscillators.

The frequency temperature characteristics of the tuning fork type quartz crystal oscillator, which consumes less power when driven, are shown as a quadratic curve in Figure 1. Therefore, when designing clock errors, in addition to the frequency tolerance at room temperature (+25 ℃), the frequency temperature characteristic tolerance of the quadratic curve should also be considered.

Assuming that a typical tuning fork type quartz crystal oscillator is continuously operated for one month at -40 ℃, its oscillation frequency tolerance will reach around -150 × 10-6, which is equivalent to a time error of more than 6 minutes (month difference of 6 minutes).

For this reason, designers may consider using oscillators with good frequency and temperature characteristics such as AT type quartz crystals as wave sources. However, the oscillation frequency of AT type quartz crystals usually reaches several MHz, so it is necessary to divide the frequency in the oscillation circuit to achieve the frequency used for clock operation. At this point, the current consumed in the oscillation circuit will reach hundreds of times that of using a tuning fork type quartz crystal oscillator. Therefore, we believe that using AT type quartz crystals as clock sources does not meet market requirements。

【Frequency accuracy compensation method using digital TCXO】

As shown in Figure 1, the tuning fork type quartz crystal oscillator has the characteristic that the oscillation frequency varies with the surrounding temperature. To improve clock accuracy, precision compensation is required. Figure 2 shows the frequency accuracy compensation method used by Epson through digital TCXO temperature compensation.

This method converts the surrounding temperature information into numbers at regular intervals, retrieves the corresponding compensation value for that temperature from memory, and compensates for the oscillation frequency. There are two main types of oscillation frequency compensation methods: capacitor adjustment method and logic adjustment method. The Epson real-time clock module mainly uses capacitor adjustment method. Explain the compensation method on the next page.

<Capacitor adjustment method>

The capacitance adjustment method refers to the method of compensating by changing the oscillation frequency of quartz. This method utilizes the characteristic of the oscillation frequency changing with the increase or decrease of the oscillation load capacitance of the quartz crystal oscillator to compensate for the frequency variation caused by the surrounding temperature. The simple principle is illustrated in Figure 3.

The left side of Figure 3 shows the frequency temperature characteristics of a tuning fork type quartz crystal oscillator, and the right side of Figure 3 shows the capacitance adjustment characteristics of frequency as a function of load capacitance. The specific content of compensation includes calculating the frequency variable ② based on the surrounding temperature ①, and deriving the corresponding load capacitance variable ③ for this frequency variable ②. After calling out the load capacitance variable corresponding to the temperature as the compensation value, compensate for the oscillation frequency. This method directly compensates for the oscillation frequency, so the oscillation output of the real-time clock module can be compensated to high accuracy and used as a low-frequency sleep clock.

<Logical adjustment method>

The logic adjustment method refers to the method of starting oscillation without adjusting the frequency of the quartz crystal oscillator, and compensating by adding or subtracting pulses in a part of the frequency divider circuit. The simple principle is illustrated in Figure 4. The specific content of compensation includes calculating the frequency variable ② based on the surrounding temperature ①, compensating for the corresponding frequency of the frequency variable ② in the frequency divider circuit, and outputting it.

As shown in Figure 4, it usually takes 32768 pulses to generate a "1-second" signal. If it is changed to 32767 pulses to generate a "1-second" signal, the 1-second cycle can be shortened. If this compensation is performed at a frequency of once per second, the frequency compensation amount is equivalent to about 30.5 × 10-6. By adjusting the number of pulses generated for 1 second and changing the compensation frequency, significant compensation can be achieved without altering the oscillation circuit. Moreover, this method uses logic circuits for adjustment, ultimately resulting in the correct output of a 1-second signal for rotating the clock. Therefore, it is widely used as a clock source for watches and other devices that operate in 1 second. However, the period of the clock signal output to the outside changes sharply due to temperature compensation, which will cause the CPU using the clock to not work at the correct time. When using this compensation method, surrounding components will not be able to enjoy its benefits.

Figure 5 shows the frequency temperature characteristics of the Epson real-time clock module using digital TCXO for frequency accuracy compensation (capacitor adjustment method).

Figure 5: Frequency temperature characteristics before and after compensation using digital TCXO

Compared with the temperature characteristics of the tuning fork type quartz crystal oscillator (green line in Figure 5), it can be seen that the compensated temperature characteristics of the Epson real-time clock module (blue line in Figure 5) remain stable over a large temperature range, with a clock error of only 9 seconds (frequency accuracy of ± 3.4 × 10-6), achieving high precision and stability.

【Epson's built-in digital TCXO real-time clock module product introduction】

Epson's real-time clock module products achieve excellent frequency accuracy after frequency compensation using digital TCXO, and have the characteristics of high precision, high stability, and low power consumption.

Table 1 shows the characteristics and summary of Epson products.

Table 1: Introduction to high-precision real-time clock module with built-in digital TCXO

Epson's products not only have excellent frequency stability, but also come in two sizes: LC and SA, making them essential for real-time clock modules.

In summary, Epson provides high-precision and low-power real-time clock module products to the market through its production technology of tuning fork type quartz crystal oscillators with low power consumption advantages and compensation circuit technology for frequency and temperature characteristics. In addition, our products adjust the frequency accuracy before leaving the factory, which is provided to customers on the basis of ensuring the accuracy. Therefore, we do not need to adjust the frequency when using, making great contributions to customers to improve design efficiency and product quality.