Need for series resistors for OCTOSPI on STM32H7- and U5-boards?

[Southbranch]

Hi,

I am working with some custom boards for H7 and U5-series.

The board schematics for STM32H735IG-DK and STM32U5A9-DISC shows both boards are using the same OCTO SPI but only the H7-board is using series resistors of 33 ohms for the all the data lines. 

In order to minimize reflections and match the transmission’s 50 ohms impedance, I have understood it as series resistor values should be chosen as: R_value = 50 ohm -  MCU_impedance. In such case one would assume the H7-series has an output impedance of about 20 ohms and the U5-series about 0 ohms.

• Is this a correct assumption / best practice?
• Are series resistors needed/reccomended on a U5-design?
• Is output impedance documented somewhere? (could not find it in ref. manuals for memory bus)

Thanks for any hints

[FBL · Accepted solution]

Hello @Southbranch 

It is recommended to use series resistors on high-speed data lines to minimize reflections and match the transmission line impedance. On U5, it depends on the characteristics of the transmission line and the output impedance under specified conditions as specified in stm32u5 datasheets. Generally, they are not needed as in our reference boards since we have low output impedance.

Some general guidelines are detailed in 9.4.3 Octo-SPI interface Getting started with STM32H723/733, STM32H725/735 and STM32H730 Value Line hardware development -

You can find more PCB guidelines in memory manufacturer app notes.

It is recommended to enable IO Compensation cell and set io speed very high for better accuracy and stability in internal voltage.