pico-platform/docs/timing/precision-timing-concepts.md

Precision Timing Concepts

Time Synchronization Protocols

NTP (Network Time Protocol)

• Accuracy: Typically 1-10ms over Internet, <1ms on LAN
• Protocol: UDP port 123
• Versions: NTPv3 (RFC 1305), NTPv4 (RFC 5905)
• Stratum Levels: 0 (reference) to 15
• Key Features:
  • Client/server and peer modes
  • Symmetric key authentication
  • Broadcast/multicast support

PTP (Precision Time Protocol)

• Standard: IEEE 1588-2008 (PTPv2)
• Accuracy: Sub-microsecond with hardware support
• Transport: Ethernet (Layer 2) or UDP (Layer 3)
• Clock Types:
  • Ordinary Clock (OC)
  • Boundary Clock (BC)
  • Transparent Clock (TC)
• Messages:
  • Sync, Follow_Up
  • Delay_Req, Delay_Resp
  • Announce, Management

IRIG Time Codes

• Types: IRIG-A, B, D, E, G, H
• IRIG-B: Most common (100 pps, 1kHz carrier)
• Encoding: Amplitude modulated or DC level shift
• Information: Time of year, year, status bits
• Accuracy: Microsecond level

GPS Time

• Source: Global Positioning System satellites
• Accuracy: ~40ns relative to UTC
• Outputs:
  • NMEA sentences (serial data)
  • PPS (Pulse Per Second)
  • 10MHz reference (some receivers)
• Considerations:
  • GPS-UTC offset (leap seconds)
  • Antenna placement
  • Signal acquisition time

PPS (Pulse Per Second)

• Signal: Rising edge aligned to second boundary
• Accuracy: Typically <100ns jitter
• Uses:
  • Clock discipline
  • Frequency reference
  • Time interval measurements

Clock Synchronization Algorithms

Phase-Locked Loop (PLL)

• Adjusts local clock frequency
• Minimizes phase error
• Smooth corrections

Frequency-Locked Loop (FLL)

• Adjusts clock rate
• Good for initial synchronization
• Handles large offsets

Kalman Filter

• Optimal state estimation
• Handles noisy measurements
• Predicts clock behavior

Clock Discipline Algorithm

1. Measure time offset
2. Calculate frequency error
3. Apply corrections
4. Monitor stability

Time Scales

UTC (Coordinated Universal Time)

• International time standard
• Includes leap seconds
• Based on atomic clocks

TAI (International Atomic Time)

• Continuous time scale
• No leap seconds
• UTC = TAI - leap_seconds

GPS Time

• Started January 6, 1980
• No leap seconds
• GPS = TAI - 19 seconds

Precision Timing Metrics

Accuracy

• Closeness to true time
• Absolute time error

Precision

• Repeatability of measurements
• Standard deviation

Stability

• Allan deviation
• Time deviation (TDEV)
• Maximum time interval error (MTIE)

Jitter

• Short-term variations
• Peak-to-peak or RMS

Wander

• Long-term variations
• Low-frequency drift

Implementation Considerations

Hardware Timestamping

• PHY or MAC layer stamps
• Reduces software latency
• Critical for PTP

Interrupt Latency

• PPS edge detection
• Deterministic response
• Priority management

Clock Sources

• Crystal oscillators (XO)
• Temperature compensated (TCXO)
• Oven controlled (OCXO)
• Chip scale atomic clocks (CSAC)

Network Considerations

• Asymmetric delays
• Packet delay variation
• Queue management
• Traffic prioritization