LIN Bus: Preparation, Placement and Process Control

A LIN bus is a low speed single wire network used inside a vehicle, mostly to connect switches, sensors and small actuators to a body controller. The transceiver is the interface between the protocol controller and the wire, and it has to survive the electrical environment of a car.

The Physical Layer

One wire carries the data and it is referenced to the vehicle ground. The signal is dominant low and recessive high, and the idle state is the recessive level close to the battery voltage, which is produced by a pull up resistor in the master node.

The bus is bidirectional and half duplex, so the same wire carries the request from the master and the answer from the slave. Collisions are avoided by the protocol, which never allows two nodes to transmit at once.

The bus voltage follows the battery, which varies from about eight volts during a cold crank to sixteen volts while charging. A transceiver that is powered from the regulated rail has to handle a bus at a different potential, and the design has to keep that difference within the ratings.

Transceiver Structure

The transceiver contains a driver that pulls the line low, a receiver that compares the line with a reference, and a level shifting stage that connects to the logic supply of the controller.

The receiver threshold is usually set at about half the battery voltage, which gives the largest noise margin against the ground offset between nodes. Some parts have a threshold that follows the supply, which is simpler but less tolerant of offsets.

A pull up resistor in the master node holds the bus recessive when no node is driving. The value is a compromise between the rise time and the current drawn when a node pulls the line low, and it is specified in the standard.

slew rate and Emission

The standard limits the slew rate of the bus edges, which keeps the radiated emission low and reduces the reflection on a long harness. The transceiver controls the slope internally rather than leaving it to the external circuit.

A slow edge limits the maximum data rate. LIN runs at up to twenty kilobaud, which is slow enough that a heavily controlled edge is acceptable, and the resulting emission is far below what a fast edge would produce.

Slow edges make the bus sensitive to noise during the transition, because the receiver is between its thresholds for longer. The high level of the bus and the hysteresis of the receiver together keep the link reliable in the presence of the noise of a vehicle.

bus termination and the Harness

A LIN bus is not terminated with resistors at both ends in the way that a high speed differential bus is. The master has a pull up resistor and a series diode, and the slaves have only their bus pin capacitance.

The capacitance of the harness sets the rise time together with the pull up resistor. A long harness with many nodes has a slow rise, and the value of the pull up has to be chosen so that the edge settles within the bit time.

Each node adds capacitance and leakage, and the standard limits the number of nodes for that reason. A design that adds a node without checking the total capacitance finds that the link works on a short harness and fails on a long one.

Protection and the Automotive Environment

The bus pin has to survive a short to the battery, a short to ground and load dump transients. The transceiver includes a clamp for the negative excursions and a current limit for the positive ones.

The supply pin needs reverse polarity protection. A misconnected battery applies minus twelve volts to the module, and a series diode or a P channel switch protects the transceiver from a condition that is common in service.

The logic side has to be protected from the bus side as well. A fault that raises the bus beyond the battery voltage must not reach the controller, which is the reason the transceiver is a separate component rather than an integrated part of a microcontroller.

Sleep, Wake Up and the Supply

A LIN node in a vehicle spends most of its life asleep, and it wakes when the bus goes dominant. The transceiver therefore has a low power mode in which the receiver still works but the rest of the module is off.

The wake up current drawn from the bus has to be low enough that a large number of sleeping nodes does not load the master pull up. The specification gives a maximum, and it is one of the parameters that distinguishes an automotive part from a general purpose one.

The local wake up input lets the node wake from a switch as well as from the bus. Its input has a threshold and a filter so that a contact bounce does not produce a wake event that the protocol then has to handle.

Interaction with the Controller

The transceiver connects to a serial port of the controller, and the state of the bus appears on that port as the digital signal. The protocol timing, the checksum and the schedule are all handled in software.

The propagation delay of the transceiver has to be included in the timing budget, and it has to be stable with temperature. A delay that varies with the supply appears as a jitter in the bit boundaries.

The diagnostic output of a transceiver reports a bus fault such as a short to ground or to the battery. Reading it and reporting the condition saves hours of diagnosis in a workshop, and it costs one pin.

Verification and Faults

Verify the bus with an oscilloscope on the line at the farthest node. The recessive level, the dominant level and the rise time are the three numbers to record, and they show whether the pull up and the capacitance are as designed.

Check the behaviour with the bus shorted and with the supply at both extremes. A transceiver that survives a short to the battery and recovers when it is removed is doing exactly what the application requires.

A link that fails only on one vehicle points to a harness or a ground offset problem rather than to the transceiver. Measuring the potential between the grounds of two nodes explains the failure in a few minutes. The release checks that keep such a module consistent are collected in our PCB design release checklist, the assembly points in judging PCB quality, and the layout measures that keep a noisy interface quiet in our guide to EMI suppression design principles.

FAQ

Why is a LIN bus not terminated at both ends? It is a single wire bus with a pull up in the master. The capacitance of the harness and the pull up value set the rise time.

What limits the number of nodes? The total bus capacitance and the leakage current of the nodes. Adding one without checking finds the limit empirically.

Why does my node fail to wake? The wake up current or the threshold. Check the bus level at the node and the specification of the low power mode.