topic Re: Cannot write registers in TJA1101B from MPC5777c in MPC5xxx

Cannot write registers in TJA1101B from MPC5777c

NickM — Fri, 23 Jun 2023 20:24:03 GMT

I have a custom PCB with an MPC5777C communicating with a TJA1101B. So far I have been able to read registers but I have been unable to write to them.

I can confirm the reads are successful by reading out the OUI from register 2 that returns a value of 0x180 and matches the default value in the datasheet.

Would there be any lock out function that is preventing me from writing to the registers that I would need to disable first?

The second question is I noticed that MPC5777C is configured to send out 5V signals on MDC and MDIO. I looked at the datasheet for the TJA1101B and it does not list any maximum voltages that it can tolerate on those two pins. Could it be related to not being able to write to registers?

Re: Cannot write registers in TJA1101B from MPC5777c

lukaszadrapa — Mon, 26 Jun 2023 13:51:16 GMT

Hi @NickM

let me copy here some screenshots from Application Hint AH1706:

The specification of MDC/MDIO pins seems to be confusing. Not sure why the "max" spec for VIH is missing but you can't connect 5V here. VDDIO is 3.3V, so these pins must be connected to pins with the same voltage level.

Regards,

Lukas

ThankRe: Cannot write registers in TJA1101B from MPC5777c

NickM — Mon, 26 Jun 2023 14:08:06 GMT

Lukas,

Thank you for the quick response.

I have tried writing to that register with no success. I suspect my issue is the 5V vs 3.3V in this scenario.

Nick

Re: Cannot write registers in TJA1101B from MPC5777c

NickM — Tue, 27 Jun 2023 14:27:59 GMT

@lukaszadrapa after changing the TJA1101B and the I/O voltage to 3.3V I am able to write registers. I was able to change the voltage for the FEC I/O and replace the TJA1101B.

I was looking at which pin to run 3.3V to only set the FEC to 3.3V and leave the rest of the I/O at 5V. Based on the information I can find it looks like I need to tie AF2 (VDDE2A) to 3.3V to do that.

Can you confirm this or let me know if there are additional pins that need to be 3.3V?

Re: Cannot write registers in TJA1101B from MPC5777c

lukaszadrapa — Wed, 28 Jun 2023 12:19:50 GMT

Hi @NickM

If you open IO definition file which is attached inside the reference manual, you can see which power domain is used for a pin:

Regards

Lukas