https://community.nxp.com/t5/Other-NXP-Products/TJA1121-Difference-between-DEVICE-CONTROL-START-OPERATION-and/m-p/2336767#M31355 <P>From the TJA1121 AN4517, the last step of PHY initialization is DEVICE_CONTROL_START_OPERATION=1.&nbsp; However, the demo code didn't write to 0x40 but instead write to register 0x8100 (PHY_CONTROL) to start PHY operation.&nbsp; What's the difference between the two registers?&nbsp; Now the PHY_STATUS MM30.0x810C=0x10A (i.e. link is in sync state but Transmit state is SEND_Z).&nbsp; &nbsp;But the same code on EVK works fine.&nbsp; Why the transmitter state is SEND_Z?&nbsp; What debug steps do you suggest?&nbsp; Thanks.</P><P>Config:</P><UL><LI>Config6=open: autonomous lead</LI><LI>I haven't started data transfer from MCU to TJA1121 via SGMII yet</LI><LI>Device confirmed not in reset (via reset pin) or low power mode (8181h is 0x0000)</LI></UL> Fri, 20 Mar 2026 17:15:06 GMT peter123 2026-03-20T17:15:06Z https://community.nxp.com/t5/Other-NXP-Products/TJA1121-Difference-between-DEVICE-CONTROL-START-OPERATION-and/m-p/2336767#M31355 <P>From the TJA1121 AN4517, the last step of PHY initialization is DEVICE_CONTROL_START_OPERATION=1.&nbsp; However, the demo code didn't write to 0x40 but instead write to register 0x8100 (PHY_CONTROL) to start PHY operation.&nbsp; What's the difference between the two registers?&nbsp; Now the PHY_STATUS MM30.0x810C=0x10A (i.e. link is in sync state but Transmit state is SEND_Z).&nbsp; &nbsp;But the same code on EVK works fine.&nbsp; Why the transmitter state is SEND_Z?&nbsp; What debug steps do you suggest?&nbsp; Thanks.</P><P>Config:</P><UL><LI>Config6=open: autonomous lead</LI><LI>I haven't started data transfer from MCU to TJA1121 via SGMII yet</LI><LI>Device confirmed not in reset (via reset pin) or low power mode (8181h is 0x0000)</LI></UL> Fri, 20 Mar 2026 17:15:06 GMT https://community.nxp.com/t5/Other-NXP-Products/TJA1121-Difference-between-DEVICE-CONTROL-START-OPERATION-and/m-p/2336767#M31355 peter123 2026-03-20T17:15:06Z