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STM32U585 I2C SCL remains low and I2C hangs

Associate II

I have two STM32L0 (slave) and STM32U585 (master) MCUs. I have no issue when Slave transmits data to Master. But, if Master transmits signal to Slave, it sends all the 9 clock pulses with address bits, but then the SCL remains low while SDA returns high.
I see my code hangs at this point and never continues, until I restart the MCU:

*/* Write data to TXDR */
hi2c->Instance->TXDR = *hi2c->pBuffPtr;*

This is located in HAL_I2C_Master_Transmit() function.
It seems to be a common issue with either STM32 or HAL libraries. Wondering if anyone could find a fix for that.

Pavel A.
Evangelist II

There's something confusing in your description. How the slave can transmit without the master addresses it first?

my code hangs at this point and never continues

Please break into the debugger (click the "pause" icon). Where it will stop?


I pointed to two tasks I did

1- Master receive data from Slave => I had no issue with this part. Basically, Master provides clock and address to slave, then slave puts data right on the clock edges.

2- Master transmits data to Slave => This is expected to be much simpler. However, When my Master transmits clk and address successfully, it does not continue to transmit data. The SCL line remains low after 9 pulses (for address transmission) while SDA gets back to high.

When I pause the debugger, it stops at nowhere. Right after executing that line, I see described signals on my oscilloscope and the debugger hangs there.

From this testing and debugging, I can conclude that there are no hardware-related causes such as pull-up resistors, wiring, etc. Because the other direction works fine.

David Littell
Senior III

This sounds like a slave-side problem with writes.

Slave is receiving data only. I just have this command in my Master:

HAL_I2C_Master_Transmit(&hi2c3, (uint16_t)I2C_ADDRESS, (uint8_t *)aTxBuffer, 2, 100);

and this one in my Slave:

HAL_I2C_Slave_Receive(&hi2c1, (uint8_t *)aRxBuffer, 2, 100);

the problem is Slave is not blocked but my Master execution is totally blocked on that line and SCL line remains low.

Do you have any suggestion for debugging further? I tried different number of data, but it had no impact.