The real-time clock module RX8900CE meets the accuracy requirements of intelligent door locks

Apr 10,2024
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Smart door locks are a type of smart home product that has emerged in recent years. Compared with old-fashioned mechanical door locks, smart door locks not only introduce novel door opening methods such as passwords, fingerprints, and facial recognition, but also can perform some intelligent management. For example, adding door opening records, setting expiration dates for passwords, fingerprints, and other users, as well as setting vacation modes and other functions. The implementation of these functions requires a real-time clock circuit in the door lock.

In practical applications, due to differences in geography, season, and external environment, the working environment temperature of smart door locks may vary greatly. Generally, smart door locks are required to work reliably in environments ranging from -25 ° C to+70 ° C. At present, some real-time clock modules used in smart door locks are all externally mounted with a regular 32.768kHz crystal. The frequency stability of ordinary crystals is poor and easily affected by temperature changes, making it difficult to ensure the accuracy of real-time clocks, resulting in some smart door locks not being able to maintain good timing accuracy when working at room temperature. In some northern regions, the winter environment temperature is very low, and there is a large timing deviation, which may lead to inaccurate door opening record time, and may also cause the validity period of the time sensitive password set for door locks to be advanced or delayed, making it impossible for users to open the door. The real-time clock module RX8900CE launched by EPSON can effectively solve this problem. Its built-in crystal can effectively solve the problem of accuracy deviation (external crystals can easily cause accuracy deviation problems in quality, crystal circuit layout, and improper introduction of matching circuits). In addition, the RX8900CE is equipped with DTCXO, which improves clock accuracy through temperature compensation.

Taking the RX8900CE UC sub model as an example, its accuracy can reach 5ppm within the temperature range of -30 ° C to+70 ° C, with a daily clock deviation of only ± 0.432 seconds and a monthly clock deviation of only+13 seconds, fully meeting the clock accuracy requirements of smart door locks in all scenarios.

The common smart door locks are usually powered by four No. 5 batteries. When the battery is almost depleted, the power supply is unstable, which may lead to large real-time clock errors. In addition, when replacing the smart door lock with a new battery, the door lock will be completely powered off for a period of time, and the real-time clock time will be lost, resulting in incorrect time information after re powering on. If the validity period is set for passwords and other permissions, replacing the battery of the door lock and powering it on will result in the inability to open the door. The RX8900CE has a backup power supply pin that supports backup power supply, which can meet the requirements of maintaining time information and accuracy for smart door locks when the main power supply is depleted or users replace the battery. The RX8900CE has a built-in power switching circuit, eliminating the need for a diode for parallel current and simplifying the circuit design.

In addition, the typical power consumption for timing RX8900CE under 3V power supply conditions is 700nA. As shown in the table above, button batteries are commonly used as backup power sources on smart door locks. During the installation and use of the smart door lock, it is not convenient to replace this battery. Taking a small CR1220 button battery as an example, its battery capacity is about 40mAh, which can provide 79 months of backup power for smart door locks, about 6 years. This ensures that under normal circumstances, the button battery does not need to be replaced throughout the entire lifecycle of the smart door lock.