first and compiling freertos

docs/timing/precision-timing-concepts.md

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+# 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