STM32L562 I2C Holding SDA/SCL at 0.8V and 1.0V respectively

[thayden]

Hello - I recently took a working design and upgraded from the STM32L4 to the STM32L5. I'm having a major issue getting the i2c bus to work properly.

• I have SCL (PB11) and SDA (PB10) configured.
• I have pull-up resistors (4.7 kohm) on each of SDA/SCL. I've confirmed that it is 3.3V on the outside of thoes resistors.
• I've configured this in STM32CubeMX to be I2C2 (I've attached the .ioc file)
• I'm almost certain the clock configuration is correct (attached the .ioc file)
• When I disconnect the STM32L5, each bus properly pulls up to 3.3V.
• However, when connected to the STM32L5, it immediately pulls SDA to 1.0V and SCL to 0.8V despite it being 3.3V on the other side of my pull-up resistors.
• I've tried everything I can think of in firmware (stm32duino and STM Cube IDE) to fix this, but there isn't a single thing that works.
• Trustzone is disabled (see option bytes below)

Here is the design, I've highlighted PB10 and PB11. Not visible in this is the 4.7 kOhm pull up resistors, but you'll have to trust me, they are there (just in a different part of the design)

Here's a picture of the board with the pull-up resistors visible:

And here's my pin configuration for PB10/11

I've also attached a copy of a simple Cube IDE program you can compile and run which performs an i2c scan. However, the scan files because both SDA/SCL remain at 0.8V and 1.0V the whole time and are never allowed to pull high. These numbers don't even make sense - why 0.8V and 1.0V?!

Things I've tried:

• Different firmware (STM32duino vs Cube IDE) - exact same behavior
• Treating PB10/PB11 as standard GPIO pins and trying to pull high (nothing happens)
• Removing all the peripherals from the custom board, makes no difference
• Removing the MCU (this causes the SDA/SCL lines to pull up back to 3.3V properly)
• Rolling back to our almost identical L4 board (which works fine with I2C!)
• Confirming the gerbers (on our custom board) are proper and not shorted and indeed connected to PB10, PB11

I don't know what more to do or what more to look into. I've read the errata for L5 and nothing stands out. Trustzone is definitely disabled, so I can't see why anything would override it here. I would appreciate any tips or leads because this is now completely blocking our project. Thank you in advance!!

My option bytes are below:

OPTION BYTES BANK: 0

   Read Out Protection:

     RDP          : 0xAA (Level 0, no protection)

   BOR Level:

     BOR_LEV      : 0x0 (BOR Level 0, reset level threshold is around 1.7 V)

   User Configuration:

     nRST_STOP    : 0x1 (No reset generated when entering Stop mode)
     nRST_STDBY   : 0x1 (No reset generated when entering Standby mode)
     nRST_SHDW    : 0x1 (No reset generated when entering the Shutdown mode)
     IWDG_SW      : 0x1 (Software independent watchdog)
     IWDG_STOP    : 0x1 (IWDG counter active in stop mode)
     IWDG_STDBY   : 0x1 (IWDG counter active in standby mode)
     WWDG_SW      : 0x1 (Software window watchdog)
     SWAP_BANK    : 0x0 (Bank 1 and bank 2 address are not swapped)
     DB256        : 0x1 (256Kb dual-bank Flash with contiguous addresses)
     DBANK        : 0x1 (Dual bank mode with 64 bits data)
     SRAM2_PE     : 0x1 (SRAM2 parity check disable)
     SRAM2_RST    : 0x1 (SRAM2 is not erased when a system reset occurs)
     nSWBOOT0     : 0x1 (BOOT0 taken from PH3/BOOT0 pin)
     nBOOT0       : 0x1 (nBOOT0 = 1)
     PA15_PUPEN   : 0x1 (USB power delivery dead-battery disabled/ TDI pull-up activated)
     TZEN         : 0x0 (Global TrustZone security disabled)
     NSBOOTADD0   : 0x100000  (0x8000000)
     NSBOOTADD1   : 0x17F200  (0xBF90000)
     BOOT_LOCK    : 0x0 (Boot based on the pad/option bit configuration)

   Write Protection 1:

     WRP1A_PSTRT  : 0x7F  (0x803F800)
     WRP1A_PEND   : 0x0  (0x8000000)
     WRP1B_PSTRT  : 0x7F  (0x803F800)
     WRP1B_PEND   : 0x0  (0x8000000)
OPTION BYTES BANK: 1

   Write Protection 2:

     WRP2A_PSTRT  : 0x7F  (0x807F800)
     WRP2A_PEND   : 0x0  (0x8040000)
     WRP2B_PSTRT  : 0x7F  (0x807F800)
     WRP2B_PEND   : 0x0  (0x8040000)

[TDK]

With NRST held low, are they still at 0.8 V / 1.0 V?

If initialized as gpio outputs, can they be set to 0 / 3.3 V?

[thayden]

Thank you for responding so quickly!

• When I hold NRST low, they still show 0.8 V / 1.0
• When initialized as GPIO outputs, I can try to set them to 0V or 3.3V but they still show 0.8 V / 1.0V

The only thing I can think of is that my PCBA shorted some pins under the MCU or something, but I can't think of which pins would show 0.8V or 1.0V to be able to even check. None of the neighboring pins provide any clues, so I'm not even sure how to complain to my PCBA

The chip I am using here is the: STM32L562RET6

[TDK · Accepted solution]

> STM32L562RET6

Close! Actually it’s STM32L562RET6Q. you can see the marking in your photo.

Sounds like you have an SMPS chip. Should be a Q at the end of the part number. These pads are not PB10/PB11 but rather SMPS pads.

[thayden]

OMG my PCBA put on the wrong chip 🤦 You are 100% correct. Thank you so much.