I am currently designing a custom board based on the STM32H753 microcontroller, and I am studying ST’s evaluation boards as reference designs for the external SDRAM interface. While comparing the schematics of MB1246 and MB1331, I noticed a discrepancy in the series resistors used on the SDRAM data bus:
On the MB1246 board, the PH8 pin (connected to D15 of the SDRAM data bus) and several other data lines include 33 Ω series resistors.
On the MB1331 board, the same positions are populated with 0 Ω links instead.
Could you please clarify the following:
What was the rationale behind using 33 Ω resistors on MB1246 versus 0 Ω on MB1331?
Does this reflect differences in PCB layout, stack-up, or signal integrity validation between the two boards?
For custom board designs using STM32H7 with external SDRAM, which approach would ST recommend:
Always include 33 Ω series resistors,
Always use 0 Ω links, or
Provide footprints for both and determine values during validation?
Your guidance will help me finalize the SDRAM interface design in alignment with ST’s best practices.
Thanks,
Mohammed
