1.12 s12: A trade-off exists between noise immunity and carrier sense latency

Description

When configured to work in noisy environments, such as those required to pass standard EMI/EMC tests, it is necessary to use additional filtering, which increases carrier sense latency. The recommended default configuration, which is required to pass these tests, results in a longer carrier assertion time, which imposes a limitation on the usable range of the PLCA Transmit Opportunity Timer. As a reminder, this value must be configured equally among all nodes in the mixing segment. Values below the default can, theoretically, provide insignificant increases in throughput at the cost of system robustness.

End User Implications

• The default value, in the specification and for this device, is 3.2 us (TO_TMR = 32). Using this value is strongly recommended except when collisions are detected on a PLCA enabled multidrop mixing segment containing third party devices.
• This device should never be configured with TO_TMR < 29, and values less than 32 require the user thoroughly test the final system for robustness.

Work Around

The Transmit Opportunity timer (TO_TMR) should be configured with the default value of 32. In the event that a smaller value is used, it must always be 29 or greater.

If collisions are detected during operation with devices from third-party vendors on the mixing segment, the Transmit Opportunity Timer may need to be increased to greater than 32. As the necessary value will be dependent upon various latency characteristics of other devices on the mixing segment, as well as the propagation delay on the physical medium, contact Microchip support.

Resolved with the release of Silicon Rev D0.