Choosing the right time source is one of the most important design decisions in any system that relies on accurate time. Depending on your requirements, you may use a simple RTC, an atomic clock receiver, GPS or internet time.
Each method has its strengths and limitations. This guide helps you select the best option based on your specific application.
Key Factors to Consider
Accuracy Requirements
- Basic timing: seconds per day acceptable → DS1307
- Moderate accuracy: seconds per month → DS3231
- High accuracy: long-term precision → atomic clock or GPS
Connectivity
- No internet available → RTC, atomic clock or GPS
- Internet available → NTP is easiest
Environment
- Indoor, noisy environment → RTC or WWVB-BPSK (ES100)
- Outdoor or clear sky → GPS
- Europe vs North America → different radio standards
Power Consumption
- Ultra-low power → RTC
- Moderate → atomic clock receiver
- Higher → GPS, WiFi (NTP)
Comparison of Time Sources
| Time Source | Accuracy | Offline | Power | Complexity |
|---|---|---|---|---|
| DS1307 RTC | Low | Yes | Very low | Very low |
| DS3231 RTC | Medium | Yes | Very low | Low |
| Atomic Clock (AM) | Very high | Yes | Low | Medium |
| WWVB-BPSK (ES100) | Very high | Yes | Low | Low |
| GPS | Extremely high | Yes | High | Medium |
| NTP | High | No | Medium | Low |
Typical Design Strategies
RTC Only
- Simple and low cost
- Used for basic timing tasks
RTC + Periodic Synchronization
- RTC keeps time continuously
- External source corrects drift
Common combinations:
- RTC + NTP (IoT devices)
- RTC + GPS (precision systems)
- RTC + atomic clock receiver (standalone clocks)
Direct Time Source Without RTC
- Possible with GPS or NTP
- Requires continuous availability
This approach is rarely used in practice.
Recommended Choices by Application
Simple Clock or Timer
- DS1307 (lowest cost)
- DS3231 (better accuracy)
Reliable Standalone Clock
- DS3231 + atomic clock receiver
Indoor Atomic Clock (North America)
- DS3231 + ES100 (WWVB-BPSK)
Outdoor Precision System
- DS3231 + GPS
IoT Device
- NTP + RTC backup
Common Mistakes
- Using DS1307 where accuracy matters
- Relying on atomic clock signals in poor reception environments
- Ignoring power consumption of GPS or WiFi
- Not combining RTC with synchronization source
Conclusion
There is no one-size-fits-all solution for timekeeping. The best choice depends on accuracy, environment, power constraints and connectivity.
In most modern designs, the best approach is to combine a reliable RTC such as the DS3231 with an external synchronization source such as NTP, GPS or an atomic clock receiver.