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Enhance RTL8221B app note with migration details

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Expanded details on the differences between RTL8221B-CG and RTL8221B-VB-CG, including LDO voltage, thermal sensor calibration, SerDes settings, and firmware readiness.
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RTL8221B-app-note.md
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@ -58,5 +58,14 @@ A unique sequence that tells the PHY to automatically drop the link speed if the
# delta between the RTL8221B-CG (2nd Gen) and the RTL8221B-VB-CG (3rd Gen)
This is vital for developers migrating their drivers.
The "VB" revision introduces architectural refinements—primarily for power efficiency and thermal stability—that change the "Magic Numbers" used in the initialization flows.The following delta is based on the transition from Application Note v2.4 to v2.5.1. The LDO Voltage Delta (Reg 0x6601)The VB revision utilizes a more efficient internal regulator. Using the old CG-revision LDO settings on a VB chip can lead to unnecessary heat or signal instability.FeatureRTL8221B-CG (Old)RTL8221B-VB-CG (New)LDO Trim (Bits 5:0)0x10 (Standard 1.1V)0x08 (Optimized for 22nm)UsageDefault for 28nm silicon.Mandatory for VB to reduce power by ~15%.Note: If you see "Bit 6" being toggled in newer SDKs, this is the LDO_Low_Power_Mode enable, which is exclusive to the VB/VM series.2. Thermal Sensor Calibration Delta (MMD 30, 0x0020)While the register address remains the same, the internal analog diode characteristics changed. Using the old formula on a VB chip will result in an error of approximately 5-8°C.Conversion Formulas:CG Revision: $T(^\circ C) = (Raw \times 0.58) - 45$VB Revision: $T(^\circ C) = (Raw \times 0.56) - 42$3. SerDes Setting Flow Delta (MMD 31, 0x6A04)The VB revision is more "picky" about the SerDes Auto-Negotiation handshake with the SoC (like the RTL9301).SettingCG RevisionVB RevisionSDS Mode Select0x0503 (Typical)0x0505 or 0x0507LogicStandard 2500Base-X.Adds enhanced SGMII-Lite support for better 2.5G sync.4. Patch/Firmware ReadinessIn the Application Note v2.5, a new check was added to ensure the PHY's internal MCU has finished its "Self-Calibration" before the MAC starts sending data.Check Register: MMD 31, 0x6A40Bit 9 (PATCH_READY): In the VB revision, you must poll this bit until it is 1 before releasing the PHY from reset. In the older CG version, a simple 10ms delay was often sufficient, but the VB's internal state machine is more complex.Migration Summary for GPL DevelopersIf you are updating a driver to support both chips, your probe function should look like this:Read MMD 30, 0x6958 (Chip Revision).If value is 0x0001 $\rightarrow$ Use CG flows (LDO 0x10, Temp Offset -45).If value is 0x0002 $\rightarrow$ Use VB flows (LDO 0x08, Temp Offset -42).
This is vital for developers migrating their drivers. The "VB" revision introduces architectural refinements—primarily for power efficiency and thermal stability—that change the "Magic Numbers" used in the initialization flows.The following delta is based on the transition from Application Note v2.4 to v2.5.
1. The LDO Voltage Delta (Reg 0x6601)The VB revision utilizes a more efficient internal regulator. Using the old CG-revision LDO settings on a VB chip can lead to unnecessary heat or signal instability.FeatureRTL8221B-CG (Old)RTL8221B-VB-CG (New)LDO Trim (Bits 5:0)0x10 (Standard 1.1V)0x08 (Optimized for 22nm)UsageDefault for 28nm silicon.Mandatory for VB to reduce power by ~15%.Note: If you see "Bit 6" being toggled in newer SDKs, this is the LDO_Low_Power_Mode enable, which is exclusive to the VB/VM series.
2. Thermal Sensor Calibration Delta (MMD 30, 0x0020)While the register address remains the same, the internal analog diode characteristics changed. Using the old formula on a VB chip will result in an error of approximately 5-8°C.Conversion Formulas:CG Revision: $T(^\circ C) = (Raw \times 0.58) - 45$VB Revision: $T(^\circ C) = (Raw \times 0.56) - 42$
3. SerDes Setting Flow Delta (MMD 31, 0x6A04)The VB revision is more "picky" about the SerDes Auto-Negotiation handshake with the SoC (like the RTL9301).SettingCG RevisionVB RevisionSDS Mode Select0x0503 (Typical)0x0505 or 0x0507LogicStandard 2500Base-X.Adds enhanced SGMII-Lite support for better 2.5G sync.
4. Patch/Firmware ReadinessIn the Application Note v2.5, a new check was added to ensure the PHY's internal MCU has finished its "Self-Calibration" before the MAC starts sending data.Check Register: MMD 31, 0x6A40Bit 9 (PATCH_READY): In the VB revision, you must poll this bit until it is 1 before releasing the PHY from reset. In the older CG version, a simple 10ms delay was often sufficient, but the VB's internal state machine is more complex.
Migration Summary for GPL DevelopersIf you are updating a driver to support both chips, your probe function should look like this:Read MMD 30, 0x6958 (Chip Revision).If value is 0x0001 $\rightarrow$ Use CG flows (LDO 0x10, Temp Offset -45).If value is 0x0002 $\rightarrow$ Use VB flows (LDO 0x08, Temp Offset -42).