Technical Explanation of I2C Issues

Issue:

Intermittent and invalid readings, including incorrect temperature values and communication option detection failures, were observed on legacy ION meters due to I2C communication issues.

Product Line:

ION8500 series
ION7550 / ION7650 series (legacy hardware revisions)

Environment:

Legacy ION meters
Firmware versions prior to corrective releases (circa 2005–2007)

Cause:

Low‑level I2C communication failures occurring within the PowerPC communications processor module (CPM) caused incomplete or malformed I2C transactions. These failures resulted in stale or invalid data being interpreted as valid sensor or I/O information by the meter firmware.

Resolution:

Symptoms

• • DSP temperature incorrectly reported as 126 °C.

Failure Mechanism

• • An I2C transaction fails to complete successfully.
  • The CPM does not report the failure.
  • Firmware reads data from the I2C receive buffer assuming it contains new data.
  • Because no new data is written, stale data from the last successful transaction is used.
  • The value 126 corresponds to the I2C write address of the I/O expander chip.

Resolution

• • • Implemented in ION8500 firmware V262 and ION7550/7650 firmware V320.
    • The I2C receive buffer is initialized with an illegal value (0x80) that cannot be returned by:
      • LM75 temperature sensors
      • I/O expander devices
    • Firmware validates buffer contents before using the data.
    • Prevents stale data from being treated as valid.

Symptoms

• • ‑97 °C displayed for DSP temperature.
  • ‑105 °C displayed for LCD temperature.
  • Communication options not detected correctly.
  • Certain communication ports and modem functionality do not operate as expected, depending on the installed communication board.

Failure Mechanism

• • Instead of returning only the I2C slave device data, the transaction returns:
    • The data and
    • The I2C slave address
  • Firmware was designed to handle data‑only returns, resulting in misinterpretation.

Resolution

• • • Patched in ION8500 firmware V262.
    • Firmware checks the length of data written to the I2C receive buffer for I/O expander transactions.
    • Correct memory location is selected based on detected return length.
    • This fix prevents communication option detection failures.
    • Temperature sensor anomalies (‑97 °C / ‑105 °C) were not corrected in this release.