Designing a clock system is only half the job. To ensure reliable performance, it is important to measure and verify its accuracy over time. This involves understanding drift, calculating ppm (parts per million) and using proper logging methods.
This article explains how to evaluate the accuracy of your clock system in a practical and meaningful way.
What Does Clock Accuracy Mean?
Clock accuracy describes how closely a clock matches the true reference time.
- Perfect clock → no deviation over time
- Real clocks → always have some error
Accuracy is typically expressed in ppm.
Understanding ppm (Parts Per Million)
- 1 ppm = 1 microsecond per second
- ≈ 0.0864 seconds per day
Example:
- 10 ppm → ~0.86 seconds per day
- 50 ppm → ~4.3 seconds per day
Drift vs Offset
Offset
- Instant difference from correct time
Drift
- Rate at which error accumulates over time
Drift is the more important parameter for long-term accuracy.
Reference Time Sources
To measure accuracy, you need a reliable reference:
- NTP (internet time)
- GPS (very high accuracy)
- Atomic clock receiver
The better the reference, the more meaningful the measurement.
Simple Measurement Method
- Set your clock to correct time
- Wait a known period (e.g. 24 hours or several days)
- Compare with reference time
Example:
- Clock loses 3 seconds in 3 days
Calculating Drift
Drift (ppm) can be calculated as:
- ppm = (time error / elapsed time) × 1,000,000
Example:
- 3 seconds error over 3 days
- 3 / (3 × 86400) × 1,000,000 ≈ 11.6 ppm
Long-Term Measurement
Short tests may not reveal true performance. Better results are obtained by:
- Measuring over several days or weeks
- Recording multiple data points
This averages out temporary variations.
Logging Methods
Manual Logging
- Record time difference periodically
- Simple but less precise
Automated Logging
- Use microcontroller to log timestamps
- Compare with reference automatically
Data Logging Example
- Store timestamp every hour
- Compare with NTP or GPS
This allows detailed analysis.
Graphing Drift
Plotting drift over time reveals patterns:
- Linear drift → stable oscillator error
- Non-linear drift → temperature effects
This helps identify underlying issues.
Temperature Testing
- Test at different temperatures
- Observe drift changes
This is especially important for crystal-based RTCs.
Comparing Clock Types
| Clock Type | Typical Drift |
|---|---|
| DS1307 | 20-60 ppm |
| DS3231 | ~2 ppm |
| GPS / Atomic | Near zero |
Improving Accuracy Based on Measurements
- Apply software compensation
- Adjust DS3231 aging register
- Add periodic synchronization
Common Mistakes
- Measuring over too short time
- Using inaccurate reference time
- Ignoring temperature effects
Best Practices
- Measure over long periods
- Use high-quality reference
- Log and analyze data
Conclusion
Measuring clock accuracy is essential for understanding and improving system performance. By calculating drift and using proper logging methods, you can identify issues and apply effective corrections.
With careful measurement and calibration, even low-cost systems can achieve surprisingly high accuracy.