External Memory Interface Handbook Volume 3: Reference Material

ID 683841
Date 7/24/2019
Document Table of Contents Partial Writes

The ECC logic supports partial writes.

Along with the address, data, and burst signals, the Avalon-MM interface also supports a signal vector, local_be, that is responsible for byte-enable. Every bit of this signal vector represents a byte on the data-bus. Thus, a logic low on any of these bits instructs the controller not to write to that particular byte, resulting in a partial write. The ECC code is calculated on all bytes of the data-bus. If any bytes are changed, the IP core must recalculate the ECC code and write the new code back to the memory.

For partial writes, the ECC logic performs the following steps:

  1. The ECC logic sends a read command to the partial write address.
  2. Upon receiving a return data from the memory for the particular address, the ECC logic decodes the data, checks for errors, and then merges the corrected or correct dataword with the incoming information.
  3. The ECC logic issues a write to write back the updated data and the new ECC code.

The following corner cases can occur:

  • A single-bit error during the read phase of the read-modify-write process. In this case, the IP core corrects the single-bit error first, increments the single-bit error counter and then performs a partial write to this corrected decoded data word.
  • A double-bit error during the read phase of the read-modify-write process. In this case, the IP core increments the double-bit error counter and issues an interrupt. The IP core writes a new write word to the location of the error. The ECC status register keeps track of the error information.

The following figures show partial write operations for the controller, for full and half rate configurations, respectively.

Figure 159. Partial Write for the Controller--Full Rate
Figure 160. Partial Write for the Controller--Half Rate

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