DisplayPort Framing Symbols and Word Alignment

In DisplayPort architectures, serialized high-speed video and audio streams require precise boundary detection at the receiver to reconstruct valid parallel data. This article explains the critical function of framing symbols in DisplayPort deserialization, detailing how these unique bit sequences allow Clock and Data Recovery (CDR) and deserializer logic to transition from raw bit synchronization to symbol lock, overcome bit-slip issues, and accurately establish parallel word boundaries.

The Deserialization Challenge in High-Speed SerDes

DisplayPort transmits parallel pixel and audio data by serializing it across multiple high-speed differential lanes. At the receiving end, the deserializer receives an unsegmented, continuous stream of raw logical bits.

While the Clock and Data Recovery (CDR) unit extracts the bit clock and establishes bit-level synchronization, it lacks the context to identify where individual words or bytes begin and end. Without a designated reference, a parallel word could be framed at any arbitrary bit offset, shifting data values and corrupting the entire transmission.

The Function of the Framing Symbol

To resolve word boundary ambiguity, the DisplayPort transmitter embeds predetermined framing symbols into the bitstream. In DisplayPort versions utilizing 8b/10b encoding (up to DP 1.4/HBR3), this is typically achieved using special control characters (K-codes), most notably the K28.5 "Comma" symbol.

The framing symbol serves several deterministic roles:

  • Unique Bit Pattern (The Comma Property): The K28.5 symbol contains a distinct sequence—0011111010 (comma minus) or 1100000101 (comma plus)—that cannot be generated across any adjacent combination of valid data symbols. This exclusivity prevents false positives during pattern matching.
  • Barrel Shifter Alignment: The deserializer contains a pattern-detection circuit and a programmable barrel shifter. As serial data passes through the deserializer, the detector scans for the unique comma pattern. Once identified, the barrel shifter locks the parallel output window to that exact bit position, locking the 10-bit word boundary.
  • Symbol Lock Verification: During the link training phase, specifically Training Pattern Sequence 1 (TPS1), the transmitter repeatedly sends K28.5 symbols. The DisplayPort receiver must achieve and verify symbol lock across all active lanes before the link can transition to higher-level training stages, such as clock recovery and channel equalization.

Active Framing and Micro-Packet Boundaries

Beyond initial link training, framing symbols maintain synchronization during active video transmission. DisplayPort organizes data into Main Stream Attributes (MSA), secondary data packets, and video streams separated by control symbols:

  • Blanking Start (BS) and Blanking End (BE): These multi-byte framing structures indicate the transitions between active display lines and blanking periods.
  • Fill and Error Recovery: Periodic insertion of framing symbols enables the deserializer to detect bit-slip events—where electrical noise causes an extra bit or missing bit—and re-align word boundaries without requiring a full link renegotiation.

In newer standards, such as DisplayPort 2.1 utilizing 128b/132b channel coding, framing blocks and distinct Sync Headers fulfill this role instead of traditional 8b/10b K-codes. Regardless of the encoding scheme, the core role of the framing symbol remains constant: acting as an unambiguous structural beacon that forces the deserializer's parallel demultiplexer to segment raw serial bitstreams into structured, valid words.