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Lord of the time
Adding real time clock information to microcontroller driven systems is problematic. As time goes by, small errors in computing routines add up. Given that users hate entering system times, a real-time element is valuable.
NXP – once Philips Semiconductors – is an old hand in the space of real time clocks, making the PCF2127 a classic. Our figure shows a basic application circuit taken from the application note.
The extra battery ensures timing continuity if system power is unavailable…
Let us start out with the most important question: the part is pretty accurate. The small version PCF2127AT promises a three PPM accuracy from -15 to 60°C, while the PCF2127T achieves the same from -30 to 80°C.
A question of complication! RTC ICs work with some kind of serial interface. Given that NXP developed the I2C protocol, the chip makes for a great I2C real-time clock IC. Furthermore, the chip can also work as an SPI real-time clock – the figures show two operating modes.
The PCF2127 makes for a great SPI real time clock…
…but can also handle the I2C protocol perfectly
In addition to providing a simple incrementing time keeper, the chip also has a calendar system. It even takes care of leap year computations, thereby allowing you to use microcontroller space for other tasks.
Finally, a timestamp function emits a periodic signal. This is useful smart meters, where periodic wake up can be handled via an interrupt generated by the real-time clock IC.
Sadly, developers who still hand-prototype the circuits won’t be happy. The IC comes in SO16 and SO20 variants, both of which are priced similarly.
Given that NXP has a long tradition of designing sensors, the Dutch could not resist adding a temperature sensor and 512 bytes of static RAM. This allows you to store a small amount of information in a remanent fashion. Finally, the chip can also supervise the battery voltage – the circuit can inform the user if the backup battery runs low.