Can i Use Realtek RTL8211F-CG with Stm32H7

[Meer]

Hi everyone,

I have a schematic using the Realtek RTL8211F-CG Gigabit Ethernet PHY and I want to connect it directly to an STM32H7 microcontroller for Ethernet communication.

My questions are:

1. The STM32H7 Ethernet MAC supports MII and RMII only. The RTL8211F-CG is a Gigabit PHY that primarily uses RGMII. Is there any way to make it work with STM32H7 without adding an intermediate chip like LAN8720 or DP83848?
2. The RTL8211F-CG datasheet mentions it can operate at 10/100 Mbps as well — but does it support a true RMII mode that the STM32H7 MAC can talk to?
3. Has anyone successfully used the RTL8211F-CG with STM32H7 directly? If so, what interface mode did you configure on the PHY side, and what changes were needed in CubeMX/HAL?
4. If this is not possible at all, what is the simplest/cheapest alternative 10/100 PHY that works reliably with STM32H7 in RMII mode?

Any help or experience is appreciated. Thanks!

[mƎALLEm · Accepted solution]

Hello,

A question that you need to address to Realtek

[Andrew Neil]

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@Meer wrote:

2. The RTL8211F-CG datasheet mentions it can operate at 10/100 Mbps as well — but does it support a true RMII mode that the STM32H7 MAC can talk to?

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You would have to ask RealTek that.

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@Meer wrote:

4. If this is not possible at all, what is the simplest/cheapest alternative 10/100 PHY that works reliably with STM32H7 in RMII mode?

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Simplest, surely, would be to use whatever the ST boards use?

[Meer]

Thanks for your comment. I will asked to RealTek

[ahsrabrifat]

No, you generally cannot connect RTL8211F-CG directly to an STM32H7 Ethernet MAC.

The problem is not the 10/100/1000 capability on the cable side. The issue is the MAC-to-PHY interface. STM32H7 Ethernet supports MII/RMII, while RTL8211F-CG is mainly an RGMII PHY. Even when the RTL8211F-CG negotiates 10 or 100 Mbps on the Ethernet cable, the MAC-side interface is still RGMII, just running at lower clock rates. It does not magically become RMII.

So unless your STM32H7 variant has an RGMII-capable Ethernet MAC, which normal STM32H7 parts do not, CubeMX/HAL cannot fix this in software. The pinout, clocking, and data bus are physically different.

For STM32H7, the simpler solution is to use a 10/100 RMII PHY such as:

• LAN8720 / LAN8742

• DP83848

• KSZ8081 / KSZ8091

• RTL8201

LAN8742 is commonly used on ST boards, so it is usually the safest choice with CubeMX examples.

In short: RTL8211F-CG is fine for processors with RGMII support, but for STM32H7 RMII Ethernet, use a proper RMII PHY.