24.4.3.3.2 HDR Double Data Rate Mode (HDR-DDR)

HDR-DDR mode uses SCL as a clock; however, unlike SDR, data and commands change on SDA on both SCL edges (when high and when low), effectively doubling the data rate. By contrast, in SDR mode, SDA is changed only when SCL is low.

HDR-DDR moves data by words. A word generally contains 16 payload bits, as two bytes in a byte stream and two parity bits. Four HDR-DDR word types are defined: Command Word, Data Word, CRC Word and Reserved Word.

Figure 24-4. Typical HDR-DDR Mode Frame Format

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The HDR-DDR Protocol precedes each 18-bit word with a 2-bit preamble, for a total of 20 bits per word.

Figure 24-5. HDR-DDR Word Formats
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The preamble:

  • Indicates the type of data that follows: either Command, Data, or CRC.
  • Allows the controller to terminate a read and determine whether the Target is willing to respond to a read.
  • Allows the Target to request a write termination.