SNAA386 November   2023 CDCE6214 , CDCE6214-Q1 , CDCE6214Q1TM , LMK00301 , LMK00304 , LMK00306 , LMK00308 , LMK00334 , LMK00334-Q1 , LMK00338 , LMK03318 , LMK03328 , LMK3H0102 , LMK6C , LMK6H , LMKDB1104 , LMKDB1108 , LMKDB1120 , LMKDB1202 , LMKDB1204

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Introduction to PCIe
    1. 2.1 The PCIe Link
  6. 3PCIe Clocking Architectures
    1. 3.1 Common Clock Architecture
    2. 3.2 Separate Reference Architecture
    3. 3.3 Spread Spectrum Clocking
    4. 3.4 PCIe REFCLK Topology
    5. 3.5 Noise Folding
  7. 4PCIe Clocking Specifications
    1. 4.1 REFCLK Output Format
    2. 4.2 PCIe Jitter Requirements
    3. 4.3 PCIe Time Domain Requirements
  8. 5REFCLK Measurement Technique
    1. 5.1 Clock Generator Measurement Results
      1. 5.1.1 PNA Measurement Result without SSC
      2. 5.1.2 PCIe Filtered PNA Result without SSC
      3. 5.1.3 PNA Measurement Result, With SSC
      4. 5.1.4 PCIe Filtered PNA Result, With SSC
      5. 5.1.5 Time Domain PCIe Measurement Result
    2. 5.2 Clock Buffer Measurement Results
      1. 5.2.1 PNA Measurement Result
      2. 5.2.2 PCIe Filtered PNA Result
      3. 5.2.3 Time Domain PCIe Measurement Result
  9. 6Texas Instruments Products with PCIe Compliance
  10. 7Summary
  11. 8References

3 PCIe Clocking Architectures

In PCIe systems, both the transmitter and receiver devices are provided with a reference clock, referred to as REFCLK. For all PCIe generations, REFCLK is a 100 MHz HCSL clock, and Table 3-1 shows the maximum frequency stability requirements by generation for Common Clock architectures.

Table 3-1 REFCLK Frequency Stability by PCIe Generation
PCIe Generation Frequency Stability (ppm)(1)
PCIe 1.1 ±300
PCIe 2.1 ±300
PCIe 3.1 ±300
PCIe 4.0 ±300
PCIe 5.0 ±100
PCIe 6.0 ±100
(1) For all Separate Reference architectures, the frequency stability of REFCLK is ±100 ppm, regardless of the bit rate.

The clocking architectures are as follows:

  • Common Clock (CC)
  • Common Clock with Spread (CCS)
  • Separate Reference No Spread (SRNS)
  • Separate Reference Independent Spread (SRIS)