STM32H7 SPI + Level Shifter

[2sayle]

Hi everyone,

I am currently in the process of developing a board that incorporates an STM32H743VIT and a voltage level shifter (FXMA108BQX) positioned adjacent to it. The objectives are to translate 3.3V logic levels to custom voltage levels (programmable) when using SPI, and to achieve at least 20MHz on this bus.

Simplified architecture: STM32H743 <-> TMUX1574 (Multiplexer) <-> FXMA108BQX <-> Connector

Problem: I am observing significant overshoots (up to 30%) on the SPI signals when probing at the connector (refer to the attached picture).

Observations:

1. I am aware that I can adjust the speed of edges on the STM32 side, which results in a clean signal up to the level shifter.
2. The level shifter appears to produce constant fast edges (approximately 2-3ns) that seems to be independent from the edge speed setting on the MCU.
3. When I program a Nucleo-64 board with the fastest edges option (VHRT), I also observe overshoots (refer to the attached pictures). No overshoots are observed if I use medium speed edges (MRT).

Questions:

1. Are the overshoots observed on my board linked to the level shifter?
2. Can I reduce them by adding a 22 or 33-ohm damping resistor?
3. Are these overshoots considered normal, considering that I am communicating at 4MHz (which is reasonably low)?

Thank you for your insights and assistance.

EJ

[Tesla DeLorean]

Pretty sure the FXMA108BQX isn't looking to follow the slew rate of the STM32, likely has Schmitt triggers with clear thresholds, and then slews with the assumption of a 80-100 MHz signalling, caring little that your application is only 4 MHz

[Billy OWEN]

Hi @2sayle 

The forum moderator had marked your post as needing a little more investigation and direct support. An online support case has been created on your behalf, please stand by for just a moment and you will hear from us.

Regards,

Billy