topic MCXN947 I3C IBI payload issue in MCX Microcontrollers

MCXN947 I3C IBI payload issue

JackieZhu — Wed, 05 Mar 2025 11:08:36 GMT

Hi,

I met a few issues about I3C receive IBI payload issue. Hope you can help. Appreciate.

When I used NXP974 as I3C master, how many payload data include MDB can we received? In my test, it seems the maximum is 8 bytes. (one MDB and seven payload data). But from the spec, it indicates that support 9 bytes include MDB. Can you help to confirm?

Spec : The controller automatically stops IBI data after 9 bytes (including the mandatory data byte).

Another issue I met is when I set the payload to 8 bytes, I do received the 8 bytes from fifo but it will enter IBI again and caused system stuck even if we didn't send new IBI from i3c target. When the payload size is less than 8 bytes, it works well.

I checked the RX FIFO count from SDATACTRL, it is 8. I think it is correct. And the status of irq seems correct, it is same as normal case (0x2e00). So I guess it is correct too.

Then, I tried to capture the waveform by LA. I found some different between correct and incorrect case.

In normal case (payload < 7) the whole clock period of last T-bit is low, then generate a Stop.

In the wrong case, the SDA will be high in the rising edge of the T-bit clock. Then, resend clock (open-drain).

Do you know what happened on the NXP? Why there are different behaviors when the payload size is different?

Re: MCXN947 I3C IBI payload issue

Harry_Zhang — Fri, 21 Mar 2025 07:21:10 GMT

Hi @JackieZhu

Yes, you are right. I3C target supports up to seven bytes of Extended IBI Data following a Mandatory Data byte.

So include MDB, the maximum is 8 bytes. (one MDB and seven payload data).

Harry

Re: MCXN947 I3C IBI payload issue

martingcavallo — Fri, 26 Dec 2025 18:49:30 GMT

Hi Jackie!

We have also faced this problem in the past and reported it to NXP. We got the confirmation that this is a bug in the NXP I3C IP block. The IBI Payload Length cannot be greater than 8 bytes, and additionally, the I3C Target SHALL use the T-Bit to indicate the End Of Data (T-Bit LOW) in the last byte. The I3C IP Block, when acting as the controller, cannot emit the STOP condition if the T-Bit is HIGH.