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Update RTL8221B application note with new details

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Added warning about AI generation and expanded on Rate Adapter importance and initialization steps.
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RTL8221B-app-note.md
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@ -1,3 +1,5 @@
Warning! AI generated.
The RTL8226B/RTL8221B SerDes Mode Setting Flow Application Note (Version 2.4) is essentially a "recipe book" for register writes. It moves away from the static descriptions found in the datasheet and provides state-machine logic that a driver must follow to ensure link stability.
Here are the specific sequences covered in that document:
@ -15,6 +17,20 @@ Since 10M, 100M, and 1000M traffic is physically sent over a 2.5G SerDes lane, t
- The Flow: The document provides the exact values for 0x6F10 and 0x6F11 (usually 0xD455 and 0x8020).
- Requirement: It details why the Rate Adapter must be enabled specifically when the MAC side is fixed at 2.5G but the Copper side is running at lower speeds.
The Rate Adapter (RA) is the most critical logic block for 2.5G PHYs like the RTL8221B and RTL8226B when they are paired with a MAC (like your RTL9301) that is "locked" at a 2.5G SerDes frequency.
Since the SerDes lane runs at a fixed physical baud rate (3.125 GHz for 2.5G), but the copper side might be linked at 10/100/1000Mbps, the Rate Adapter performs Bit Stuffing to fill the gaps. Without proper RA initialization, the MAC will see "garbage" data at lower speeds.
According to the Application Note v2.4, the initialization must happen after the SerDes mode is selected but before the final SerDes reset is released.
Step 1: Enable the RA Clock logic. You must tell the PHY to prepare the Rate Adapter buffers.
Register: MMD 31, 0x6F10, Value: 0xD455 (This is a proprietary Realtek tuning value that stabilizes the internal FIFO depth).
Step 2: Configure RA Mode and Speed
Register: MMD 31, 0x6F11, Bit 15 (RA_EN): Must be 1 to enable the adapter, Bit 5 (RA_FIX_2500): Must be 1. This forces the Rate Adapter to always output 2.5G towards the MAC, regardless of the copper link speed. Value: 0x8020 (Standard for most 2.5G designs).
Thermal Sensor Calibration and Monitoring
The Application Note provides the "Enablement Flow" that is often missing from the generic datasheet: