TWI/TWIM bus probing / bus scanning

RE: TWI/TWIM bus probing / bus scanning

Einar Thorsrud — Mon, 20 Apr 2026 13:52:37 GMT

Hi Pascal,

The problem as I see it is that configuring TWIM->TXD.MAXCNT = 0 and starting transmission with TASKS_STARTTX is not a compliant method for TWI bus scanning. The fact that no LASTTX or STOPPED event is generated when the address is ACKed indicates that the peripheral is not completing a normal, valid transfer sequence. Because of this, using the absence of EVENTS_ERROR after a timeout as confirmation of ACK is not a robust solution. (And again, it is also a potential problem on the slave side.)

That is why I would recommend using a normal TWI/TWIM transfer for probing, as done in the twi_scanner example, and evaluating whether the addressed slave ACKs the transaction.

Br,

Einar

RE: TWI/TWIM bus probing / bus scanning

Pascal Künzi — Mon, 20 Apr 2026 06:29:26 GMT

Hi Einar

Thank you for your assessment!

We were asked for a generic TWI bus probing functionality that does not transmit or read additional data. Hence, I cannot give you a specific use case but would assume, that the possible target devices withstand the probing.

Nonetheless, I agree that there are TWI slave devices that may end up in a bad state. As a result, it will be necessary to verify that the probing is actually "supported" on the target devices before using it in production.

If you are unsure about the nRF's behavior in this situation, I rather not use my suggested approach.
I would appreciate it if you could clarify or elaborate on the potential side effects on the nRF side. If there is a chance that this may cause a problem, this idea will be rejected.

Best regards,
Pascal

RE: TWI/TWIM bus probing / bus scanning

Einar Thorsrud — Fri, 17 Apr 2026 12:54:32 GMT

Hi Pascal,

Zero length read/writes are problematic. Doing like you write with a timeout before stopping may work, but I am not sure of potential side effecs. Generally, TWI/I2C bus scanning is performed for development purposes and you should expect that devices on the bus may be in a bad state afterwards. May I ask for the use case? What is the end goal? Could it be that it could be done in another way?

Br,

Einar