140 lines
3.1 KiB
Markdown
140 lines
3.1 KiB
Markdown
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# Precision Timing Concepts
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## Time Synchronization Protocols
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### NTP (Network Time Protocol)
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- **Accuracy**: Typically 1-10ms over Internet, <1ms on LAN
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- **Protocol**: UDP port 123
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- **Versions**: NTPv3 (RFC 1305), NTPv4 (RFC 5905)
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- **Stratum Levels**: 0 (reference) to 15
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- **Key Features**:
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- Client/server and peer modes
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- Symmetric key authentication
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- Broadcast/multicast support
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### PTP (Precision Time Protocol)
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- **Standard**: IEEE 1588-2008 (PTPv2)
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- **Accuracy**: Sub-microsecond with hardware support
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- **Transport**: Ethernet (Layer 2) or UDP (Layer 3)
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- **Clock Types**:
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- Ordinary Clock (OC)
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- Boundary Clock (BC)
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- Transparent Clock (TC)
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- **Messages**:
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- Sync, Follow_Up
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- Delay_Req, Delay_Resp
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- Announce, Management
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### IRIG Time Codes
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- **Types**: IRIG-A, B, D, E, G, H
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- **IRIG-B**: Most common (100 pps, 1kHz carrier)
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- **Encoding**: Amplitude modulated or DC level shift
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- **Information**: Time of year, year, status bits
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- **Accuracy**: Microsecond level
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### GPS Time
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- **Source**: Global Positioning System satellites
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- **Accuracy**: ~40ns relative to UTC
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- **Outputs**:
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- NMEA sentences (serial data)
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- PPS (Pulse Per Second)
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- 10MHz reference (some receivers)
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- **Considerations**:
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- GPS-UTC offset (leap seconds)
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- Antenna placement
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- Signal acquisition time
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### PPS (Pulse Per Second)
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- **Signal**: Rising edge aligned to second boundary
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- **Accuracy**: Typically <100ns jitter
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- **Uses**:
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- Clock discipline
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- Frequency reference
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- Time interval measurements
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## Clock Synchronization Algorithms
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### Phase-Locked Loop (PLL)
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- Adjusts local clock frequency
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- Minimizes phase error
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- Smooth corrections
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### Frequency-Locked Loop (FLL)
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- Adjusts clock rate
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- Good for initial synchronization
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- Handles large offsets
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### Kalman Filter
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- Optimal state estimation
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- Handles noisy measurements
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- Predicts clock behavior
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### Clock Discipline Algorithm
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1. Measure time offset
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2. Calculate frequency error
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3. Apply corrections
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4. Monitor stability
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## Time Scales
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### UTC (Coordinated Universal Time)
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- International time standard
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- Includes leap seconds
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- Based on atomic clocks
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### TAI (International Atomic Time)
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- Continuous time scale
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- No leap seconds
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- UTC = TAI - leap_seconds
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### GPS Time
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- Started January 6, 1980
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- No leap seconds
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- GPS = TAI - 19 seconds
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## Precision Timing Metrics
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### Accuracy
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- Closeness to true time
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- Absolute time error
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### Precision
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- Repeatability of measurements
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- Standard deviation
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### Stability
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- Allan deviation
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- Time deviation (TDEV)
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- Maximum time interval error (MTIE)
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### Jitter
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- Short-term variations
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- Peak-to-peak or RMS
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### Wander
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- Long-term variations
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- Low-frequency drift
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## Implementation Considerations
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### Hardware Timestamping
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- PHY or MAC layer stamps
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- Reduces software latency
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- Critical for PTP
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### Interrupt Latency
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- PPS edge detection
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- Deterministic response
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- Priority management
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### Clock Sources
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- Crystal oscillators (XO)
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- Temperature compensated (TCXO)
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- Oven controlled (OCXO)
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- Chip scale atomic clocks (CSAC)
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### Network Considerations
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- Asymmetric delays
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- Packet delay variation
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- Queue management
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- Traffic prioritization
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