STM32U5A5: Unexpected behavior on PA11 when used as SPI1_MISO
Subject: STM32U5A5: Unexpected behavior on PA11 when used as SPI1_MISO
Hi,
We observed unexpected behavior on STM32U5A5 devices when using PA11 as SPI1_MISO.
The issue has been reproduced on multiple STM32U5A5 devices from different production batches as well as on an STM32U5A5 evaluation board. The same software and hardware setup works correctly on an STM32U585.
Description
===========
When SPI1 is enabled and PA11 is configured as MISO, the pin appears to load the signal unexpectedly.
In our setup, the external SPI device drives the MISO line push-pull. However, the signal level reaches only about 2.9 V when active. When the SPI device is inactive (CS de-asserted), the signal drops only to about 2.3 V.
To exclude the external SPI device as the root cause, we disconnected it completely and performed a simple GPIO test:
- PA11 configured as GPIO input
- PA11 connected to 3.3 V through a 10 kΩ resistor
Expected result:
- PA11 voltage close to 3.3 V
Actual result:
- PA11 voltage only slightly above 400 mV
This behavior suggests that PA11 is being internally loaded by the MCU.
Measurements and screenshots:
Additional observations
=======================
- The issue was reproduced with different external SPI devices.
- Some SPI devices are no longer able to drive the MISO signal to a valid logic-high level when connected to PA11.
- Communication errors can occur as a result.
- USB functionality is completely disabled and should not influence the behavior.
Workaround
==========
Moving SPI1_MISO from PA11 to PA6 resolves the issue completely.
With PA6, signal levels and SPI communication behave as expected.
Questions
=========
- Is this a known issue on STM32U5A5 devices?
- Are there any known limitations or special configuration requirements for PA11 when used as SPI1_MISO?
- Is this behavior covered by any errata document?
Thank you for your support.
Best regards,
Elias Gruber
Embedded Software Developer
Otto Bock Austria GmbH
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