High-Speed Serial Interface
Why Serial Interface?
Summary of Parallel Interface Limitations
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Paradigm Shift
“Multiple slow lanes” → “One fast lane”
Key Advantages of Serial Interface
1. Minimal Pin Count
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- Communication possible with just 1 differential pair
- Bidirectional: TX pair + RX pair = 4 pins total
- Reduced package cost, enables more channels
2. Skew Problem Solution
Complexity Reduction:
Parallel: Must manage skew between 9 signals (CLK + 8 data)
- Inter-lane skew: hundreds of ps tolerance
- Requires length matching for all 9 traces
- Extremely difficult at high frequencies
Serial: Only manage skew within differential pair (TX+, TX-)
- Intra-pair skew: few ps tolerance (±5ps typical)
- Length matching for just 2 traces
- Much simpler PCB design
Common-mode noise rejection with differential signaling
Timing margin drastically improved
3. High-Speed Operation Capability
- Parallel: Difficult above 1GHz
- Serial: 10Gbps, 25Gbps, 56Gbps possible
- Data rate = Clock frequency × Encoding efficiency
4. Reduced Electromagnetic Interference (EMI)
- Differential signals reduce electromagnetic radiation - Strong against external noise - Easy FCC/CE certification5. Simple PCB Design
- Manage only 2 traces
- Reduced length matching burden
- Can reduce layer count
Challenges of Serial Interface
1. Clock Recovery
In parallel interfaces, clock is transmitted separately, but in serial, clock must be extracted from data.
Serial Data & Clock Recovery Animation
Solution: CDR (Clock Data Recovery)
- PLL/DLL-based clock recovery
- Clock synchronization through edge detection
2. Maintaining DC Balance
Risk of clock recovery failure with consecutive ‘0’s or ‘1’s
Problem: 0000000000... ─────────── (no edges)
1111111111... ────────────
Solution: 01010101... ─┐ ┌─┐ ┌─┐ ┌─ (abundant edges)
└─┘ └─┘ └─┘
Solutions:
- 8b/10b encoding: Guarantees DC balance
- Scrambler: Data randomization
3. Ensuring Data Integrity
Increased possibility of errors in high-speed transmission
Solutions:
- CRC: Error detection
- FEC (Forward Error Correction): Error correction
4. Serialization/Deserialization
Parallel → Serial Serial → Parallel
[8 bits] [1-bit stream]
┌──────────┐ ┌──────────┐
│Serializer│ ────────────► │Deserializ│
│ │ 1-bit │ er │
└──────────┘ └──────────┘
Solution: SERDES (Serializer/Deserializer)
Major Standards and Protocols
PCI Express (PCIe)
- Speed: Gen1 (2.5GT/s) ~ Gen5 (32GT/s)
- Encoding: Gen1/2 uses 8b/10b, Gen3+ uses 128b/130b
- Lanes: x1, x4, x8, x16 configurations
USB
- USB 2.0: 480Mbps
- USB 3.0: 5Gbps (8b/10b)
- USB 3.1: 10Gbps
- USB4: 40Gbps
SATA
- SATA 1.0: 1.5Gbps
- SATA 2.0: 3Gbps
- SATA 3.0: 6Gbps
- Encoding: 8b/10b
Ethernet
- 1000BASE-T: 1Gbps
- 10GBASE-R: 10Gbps (64b/66b)
- 25G/50G/100G Ethernet: 25Gbps+
DisplayPort / HDMI
- DisplayPort 1.4: 32.4Gbps (4 lanes)
- HDMI 2.1: 48Gbps
Aurora, RapidIO
- Xilinx and other FPGA manufacturer high-speed serial protocols
Serial Interface Layer Architecture
┌─────────────────────────────────────┐
│ Application Layer │ ← User data
├─────────────────────────────────────┤
│ Transaction/Link Layer │ ← Packetization, flow control
│ - Packet framing │
│ - CRC │
│ - Flow control │
├─────────────────────────────────────┤
│ Physical Coding Sublayer (PCS) │ ← Encoding/Decoding
│ - 8b/10b or 64b/66b encoding │
│ - Scrambling │
├─────────────────────────────────────┤
│ Physical Media Attachment (PMA) │ ← SERDES
│ - Serialization │
│ - Deserialization │
│ - Clock Recovery │
├─────────────────────────────────────┤
│ Physical Medium Dependent (PMD) │ ← Physical layer
│ - TX Driver │
│ - RX Equalizer │
└─────────────────────────────────────┘
Bandwidth Comparison
Parallel DDR3-1600
64-bit × 1600MT/s = 102.4 Gbps (theoretical)
Actual: 12.8 GB/s = 102.4 Gbps
Pin count: 64 data + control/clock = 80+ pins
PCIe Gen3 x16
16 lanes × 8Gbps = 128 Gbps
Actual: 128b/130b encoding = ~125.9 Gbps
Pin count: 16 × 2 (differential) × 2 (TX/RX) = 64 pins (data only)
Efficiency Analysis
- Bandwidth per pin: PCIe significantly higher
- Scalability: PCIe easily scales by adding lanes
- Transmission distance: Serial far superior
Key Technologies in Serial Interface Implementation
1. 8b/10b Encoding
- Guarantees DC balance
- Supports clock recovery
- Control code insertion
2. Scrambler
- Data pattern randomization
- EMI distribution
- Improved clock recovery
3. CRC
- Data integrity verification
- Packet-level error detection
4. SERDES
- Parallel ↔ Serial conversion
- High-speed I/O interface
- CDR (Clock Data Recovery)
Design Considerations
Signal Integrity
- Impedance matching: 100Ω differential impedance
- Loss compensation: Pre-emphasis, Equalization
- Jitter management: Jitter budget calculation
Power Consumption
Power = C × V² × f × α
High-speed serial:
- f is high but fewer switching events
- Small capacitance
→ Overall power can be better than parallel
Test and Verification
- BERT (Bit Error Rate Test): Measure bit error rate
- Eye Diagram: Evaluate signal quality
- Jitter analysis: Evaluate timing accuracy
Conclusion
High-speed serial interface has become the standard for modern high-speed communications for the following reasons:
Advantages
✓ Fewer pins ✓ Higher data transfer rates ✓ Simpler PCB design ✓ Lower EMI ✓ Excellent scalability
Challenges to Overcome
- Clock recovery
- DC balance
- Data integrity
- Serialization/deserialization
These challenges have been solved through technologies such as 8b/10b encoding, Scrambler, CRC, and SERDES.
Next: Detailed principles of 8b/10b encoding