Series resistor for high speed signals in STM32H757XIH6

[JananiRamesh]

We are planning to use STM32H757XIH6 as a soul microcontroller, so we are using 10/100Mbps Ethernet, is 22 ohm or 33ohm resistor best suited for this application. If either of this is answer , please provide why one is better than other and when.

[STackPointer64]

Hello @JananiRamesh, and welcome to ST Community!

The MB1248-H747I-D04 schematic uses 33 Ω series resistors on the RMII Ethernet signals, which supports choosing 33 Ω in designs similar to this board. While many STM32H7 references recommend 22 Ω, the 33 Ω value here reflects STMicroelectronics’ optimization based on PCB layout, trace length, and signal integrity considerations. The choice between 22 Ω and 33 Ω depends on factors like trace impedance and the balance between signal damping and rise time.

Although this schematic is for the H747, the only significant difference with the H757 is the cryptography module, so the Ethernet design recommendations remain applicable. If your design closely follows this board, using 33 Ω is recommended. Otherwise, consider signal integrity testing to decide which value suits your conditions best.

Best regards,