Talking about the timing accuracy of the RTC of the external temperature sensor

“Unless corrective measures are taken, the temperature-dependent characteristics of quartz crystals will prevent high accuracy in a wide temperature range consistent with state-of-the-art real-time clocks. This application note describes how to use an external temperature sensor and place it at the same or close to the same temperature as the quartz crystal connected to the RTC, which can significantly improve accuracy. If the application is already using the temperature sensor for some reason, only some additional firmware is needed, without adding it to the bill of materials (BOM).

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Unless corrective measures are taken, the temperature-dependent characteristics of quartz crystals will prevent high accuracy in a wide temperature range consistent with state-of-the-art real-time clocks. This application note describes how to use an external temperature sensor and place it at the same or close to the same temperature as the quartz crystal connected to the RTC, which can significantly improve accuracy. If the application is already using the temperature sensor for some reason, only some additional firmware is needed, without adding it to the bill of materials (BOM).

introduce

The real-time clock (RTC) has been around for a long time, and many Electronic systems include a function, such as keeping the calendar time, rate switching, time stamping or periodically waking up the system to initiate certain actions, such as taking some measurements. . Not all applications require the same level of accuracy, but when high accuracy is required, such as when accuracy above 5 ppm is required in applications that rely on real-time data, standard RTC cannot meet these requirements. The timing can only be as accurate as its reference. Several environmental factors such as humidity, vibration and pressure will affect the accuracy of the RTC, but it is mainly the inferior characteristics of the quartz crystal in the temperature range. If the temperature changes, it will cause deviation. Various techniques have been used to improve the timing accuracy achieved using a 32.768 kHz crystal. This application note describes how to use an external temperature sensor to be placed at or close to the same temperature as the RTC and its crystal, which can significantly improve accuracy. If the application is already using the temperature sensor, you only need to add some firmware instead of adding it to the bill of materials (BOM).

If the RTC has an electronic tuning register, the process of using this method to achieve temperature compensation will be greatly simplified. Such registers allow the clock speed to be affected by adding or subtracting counts from the oscillator divider chain. This will change the duration of one second without changing the frequency of the oscillator. NXP RTC PCF85063, PCF8563 and PCF2123 provide this function.

Problems when using RTC without temperature compensation

Sometimes you can use the built-in oscillator in the system microcontroller to complete the timing. However, in many cases, the use of RTC is inevitable. There are several advantages to using a standalone RTC:

• Lower power consumption
• Release the main system for time-critical tasks
• Higher timing accuracy

Figure 1 Frequency and temperature deviation of a typical 32.768 kHz crystal

However, even with RTC, the timing can only be as accurate as the reference used. The frequency characteristic of the crystal depends on the shape or “cut” of the crystal. The manufacturer can control the crystal switching frequency by cutting the angle of the crystal. However, having to manufacture crystals with different cutting angles increases complexity and cost. The tuning fork crystal is usually cut in such a way that the change in frequency with temperature is a parabolic curve centered at 25°C, see Figure 1. This means that the tuning fork crystal oscillator will resonate to close to its target frequency at room temperature, but the speed will slow down when the temperature rises or falls.