I recently purchased the excellent PCF8523 real time clock breakout board from Adafruit for a project. It was easy to interface with using I2C and the RTClib Arduino library. It keeps the time for up to five years even with no external power by using a coin cell battery.

The only trouble was that the clock was not terribly accurate and after a few weeks was fast by several minutes. A perfect crystal would oscillate at exactly 32.768 kHz but various factors like temperature, pressure and humidity make it faster or slower. In my case, the crystal was oscillating too quickly causing the RTC to be fast by 8 seconds per day.

After reading through the PCF8523 datasheet I found that the RTC can be calibrated by setting a value in one of the device registers. Section 8.8.3 describes a way to calculate the register value.

I do not have an oscilloscope with a high enough resolution to accurately measure the frequency for step 1. Is it possible to calculate the offset without one? The NXP application note discussing improved timekeeping has some useful tidbits beyond what was described in the datasheet. Most important is the fact that 1 second of clock drift per day corresponds to an error of 11.57 parts per million. By measuring the drift of the clock over a few days we can do some simple calculations and calibrate the RTC.

RTC offset calculation

What does “correction pulse” mean? The RTC will extend or reduce the amount of time required to consider that a second has elapsed. This is the value that gets placed in the first (least significant) 7 bits of the offset register.

• If your clock is running fast you need to write a positive value to the register. This increases the number of oscillations required for a second to elapse, thereby slowing down the clock.
• If your clock is running slow you need to write a negative value to the register, coded in two’s complement. This reduces the number of oscillations required for a second to elapse, thereby speeding up the clock.

Writing to the offset register

Since the 8th bit of the register contains the mode flag, determining the final byte that should be written to the offset register requires some bit manipulation. I’ve added a calculate method to the RTClib Arduino library that does this for you. The calibrate function can be called like this:

// Adjust for the RTC running fast
#define OFFSET 23

rtc.calibrate(PCF8523_TwoHours, OFFSET);