3.3.2 Physical Layer Issues

• Has the bus been terminated correctly (with 120-Ω) at either ends (only)? The bus must be terminated only at either ends and with a 120-Ω resistor. In other words, no more than two terminator resistors may be present on the bus, unless split termination is followed, in which case there will be two resistors on either ends. While designing a CAN bus system, it is important that the termination resistors can be enabled/disabled from outside the system enclosure. This scheme makes it easy when nodes have to be added/removed to/from the network.
• Are all CAN nodes configured for the same bit-rate? Mis-matched node bit rates would repeatedly introduce error frames on the bus. Capture the output of a node on the oscilloscope to physically verify the bit-time.
• Have you tried a lower bit-rate? Say, 50 kbps, for example? Timing issues concerning propagation delays may be caught trying a lower bit-rate. Ensure that CANBTR register has the programmed value.
• For devices with an on-chip zero-pin oscillator (INTOSCn) , are you using an external clock source or the INTOSCn? If you are using INTOSCn, do you perform temperature compensation of INTOSCn (for devices where temperature compensation is allowed)?
• Have you tried to reduce the bus length and number of nodes?
• Before the occurrence of the error condition, were any error-frames seen on the bus? This could point to timing violations or noise issues.
• How many nodes are there in the bus? (In non-self-test mode, there must be at least two nodes on the network, due to the acknowledge (ACK) requirement mandated by the CAN protocol)