Automobiles now contain hundreds of sensors to measure and report on parameters including temperature and pressure. Most of the time, these sensors are remotely located within a vehicle, far from the host microcontroller that monitors and processes sensor data. Typically, these sensors don’t connect directly to a CAN or LIN network because of the vehicle wiring overhead that’s associated with connecting to the network. However, to overcome this wiring limitation and to reduce required battery supplies, you can convert the standard three-wire LIN network to a two-wire implementation where the LIN slave nodes harvest power directly from the LIN bus master communication wire.
To successfully implement a two-wire LIN network, the slave node must be supplied with sufficient power to maintain communication at a minimum system operating voltage, typically 9V. Some key parameters affecting slave node performance in this type of implementation include:
- LIN bus power supply—meet a minimum operating voltage threshold and reduce the pull-up resistor to the smallest value possible (without exceeding the current limitation specification of the LIN driver)
- Slave node current consumption—reduce current consumption by using the lowest clock frequency that enables your application to meet functional design requirements, applying duty-cycling between low and high current operating modes and lengthening the LIN schedule table period
- Slave node buffer capacitance—the capacitor must provide sufficient charge reserve to power the slave node during a LIN frame data packet and to receive a full charge between LIN frame data transmissions. Bench tests reveal that a buffer capacitor of 47µF to 100µF is enough to maintain power to the slave node for a network operating at 19.2kbaud with a 100ms delay between LIN data frames and a 9V minimum operating battery voltage.
- LIN bus data protocol—keep the LIN bus data rate high, at least 19.2kHz, to maximize the data transfer speed. Keep the quantity of data as low as possible to minimize the duration of the dominant state on the LIN bus line. Maintain a LIN schedule table period that’s long enough for the LIN bus powered slave node to fully recharge the buffer capacitor, CVS_S, between LIN message frames.
A two-wire LIN network is ideal for low-node count networks where the system is limited to one master and no more than three slaves, where all nodes are powered on simultaneously. The two-wire network can be easily implemented with a thorough understanding of the system supply/load requirements as well as some hardware modifications.
For more details, read my two-wire LIN networking article.