ISO1050: TxD pin isn't pulled low

Hi Gerasimos,

Thanks for reaching out.

To help determine the potential cause of this issue, I have the following questions:

-Could you please share a schematic that shows how the ISO1050 is connected in the design? 

-In the schematic that you share, could you also make sure that it shows everything that is connected to the ISO1050 on the logic (left) side of the device, including the part numbers of all connected devices?

-When the observed issue occurs, does it happen during EMC testing, does it happen during powerup, or does it happen when some other action is taking place?

-When you perform this test, are you using a waveform generator or are you using an MCU to drive the TXD pin low?

-If you are using an MCU to drive the pin low, can you try using either a different MCU or a waveform generator to drive the pin low? When you switch to a different MCU or a waveform generator to drive the pin low, does the voltage at the TXD pin get driven low to 0V, or does it still only go to 2.4V?

Regards,

Kenneth

Thank Kenneth for your reply,

to address your first question.

RxD and TxD are connected to a STM32G0B1 microcontroller with a CanFD controller.

The output on the TxD is generated by STM mcu.

The MCU in external loopback mode. Meaning the RxD shouldn't effect the TxD.

Currently we are only testing the canbus firmware and hardware.  The observed output is during normal operation. So the canbus core successfully emits can messages from the canfd controller.

Currently we are setting up a different MCU, when we have some results we will share them.

kind regards,

Gerasimos

Hi Gerasimos,

Thank you for providing a schematic showing how the device is connected in your design.

In the schematic, I see that the isolated CAN transceiver in the design is labeled as "ISO1042DWVRQ1". In your initial post, you said that the device being used was the ISO1050. Can you please confirm which device is being used in your design? Are you using the ISO1050, or are you using the ISO1042?

Additionally, in your initial post, you said that the TXD pin of the ISO1050 was pulled up to 3.3V. However, in your schematic, it doesn't look like the TXD pin is connected to any pullup resistors. Instead, it looks like the RXD pin of the device is pulled up to 3.3V. Can you please confirm which pin of the device you are observing this issue on? Are you observing this issue on the RXD pin of the isolated CAN transceiver, or are you observing this issue on the TXD pin of the isolated CAN transceiver? Was the oscilloscope screenshot that you provided in your initial post showing the unexpected behavior of the TXD pin, or was it showing the unexpected behavior of the RXD pin?

Gerasimos Maroulis1 said:

Currently we are setting up a different MCU, when we have some results we will share them.

Please do share your new test results with a different MCU connected whenever you obtain them. If possible, please also see if you can perform an additional test where you try driving the pin low with a waveform generator instead of an MCU.

Regards,

Kenneth