topic Re: Unused NVT4858 CLK_FB Pin in High Performance Analog Front End Interfaces

Unused NVT4858 CLK_FB Pin

JsGraaf — Thu, 10 Apr 2025 09:50:16 GMT

Hello,

I was wondering what the spec is when the CLK_FB pin is not used. My MCU (STM32U5A9) doesn't have a feedback pin for the SDMMC interface. Do I have to connect it to prevent noise?

Kind Regards,

Joris

Re: Unused NVT4858 CLK_FB Pin

TomasVaverka — Thu, 10 Apr 2025 10:43:29 GMT

Hello Joris,

The NVT4858's CLK_FB pin is used for clock synchronization, but if your MCU does not have a feedback pin for the SDMMC interface, you can leave the CLK_FB pin unconnected. The NVT4858 is designed to handle such scenarios without causing issues.

However, to minimize potential noise, ensure that your layout follows good design practices, such as keeping traces short and well-routed and using proper grounding techniques.

BRs, Tomas

Re: Unused NVT4858 CLK_FB Pin

JsGraaf — Thu, 10 Apr 2025 13:00:34 GMT

Thank you for your swift response! I have a solid ground plane directly under the signal layers (each lines passes to the other side once), all the traces are spaced more the 0.4mm apart and the trace lenghts are within 7mm of each other (max len is 44mm). The only part of the spec I have not adhered to is the 50 ohm impedance lines (mine are around 63 ohm). I will be running the interface at 48 Mhz 4-bit mode. Will this be possible without the controlled impedance?

Kind Regards,

Joris

Re: Unused NVT4858 CLK_FB Pin

TomasVaverka — Thu, 10 Apr 2025 16:25:03 GMT

Hello Joris,

Running the interface at 48MHz in 4-bit mode with 63Ω impedance lines should be feasible. I would not expect signal integrity issues, considering your short traces and good layout.

If you ever plan to run higher-speed SDMMC modes in the future, then controlled 50Ω impedance, better trace matching and using CLK_FB might become more critical. But for 48 MHz 4-bit, your setup should work without problems.

BRs, Tomas

Re: Unused NVT4858 CLK_FB Pin

JsGraaf — Fri, 11 Apr 2025 10:04:27 GMT

Hello Tomas,

Again thank you for all your help!

Kind Regards,

Joris