Cable diagnostics for TJA1100 PHY

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Cable diagnostics for TJA1100 PHY

Contributor I

We are working with the TJA1100 as a PHY with MPC5746C.

As stated in the Datasheet and the application note for TJA1100

to run the cable diagnostics :

-> The PHY must be in normal mode and then set the Cable test bit#5 in the Extended control register (Register 17).

->Then we shall disable the link control to stop all communications.


-> Then loop untill we find the Cable test bit becomes 0 again which means the test has ended.

->And then we can get the result from the External status register (Register 25).
This the sequence as I understood, but I have some weird observation:

1 - Once I set the cable test bit#5, and to check that the value has been written correctly I read the register value before setting the cable test bit and after to found that the bit is didn't changed. As if I did not change anything.

Also this happened exactly when I cleared the Link control once I read the register to check the resetting of the bit is done or not I found that the register has the previous value before resetting the link control bit.

2 - So for that reason  when I force an open or short circuit to the wires, the results always gives that the cable is Ok.

3 - Also even if I didn't that check and let everything run without double checking every time I got that the cable result is Ok without any problems while I'm forcing the open or the short circuit.

Could you please help.

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