SPI problem example read ID from TJA1145

[Andreas85]

When trying to read the ID via SPI, I receive the following responses. 0xFD74. From data sheet 0x74 is expected. 

But if I send the service 0xFD00  again, I get a different response from TJA1145. Why is that the case? The responses repeat cyclically which means BufferRx[4] would be 0xFD74.

I would like to set the transceiver to normal mode using 0x0207. However, here as well I receive different responses but not the response I expect.

Thanks for the support! 

[ErikaC]

Hello, The SPI interface operates in full‑duplex mode, meaning that data is transmitted and received simultaneously. However, the device does not return the result of the current command within the same SPI frame. Instead, the response corresponds to the previously received command.

So, could you please try 

Frame 1:
TX (MOSI): Read command → Address 0x7E with RO = 1
RX (MISO): Undefined or previous data

Frame 2:
TX (MOSI): Dummy data (e.g., 0x0000)
RX (MISO): Valid response → Device ID value (e.g., 0x74)

[Andreas85]

Thanks for the reply. 

If I send the message as recommended, I get the following answer:

It looks like the TJA1145 is responding with following register values  (16 Bit)and  reflecting SDI on SDO. I can also see if I sent Frame 1, pull SDI to low and start with reading register 0x7E, the response comes as expected.  

According to the datasheet, SDI can be left floating after the transfer. Does the slave need a low level to reset its internal state machine?

Thanks in advance! 

[ErikaC]

What is the external pull-up value connected to SDI?

Please see AH1903 Application Hints - High speed CAN transceiver for partial networking TJA1145A section 7.4.1 SDI pin input behavior that is under secure files!

[Andreas85 · Accepted solution]

Hello, I found the roor cause. The voltage from CS was too low due to the hardware configuration. Thanks for the support!