LIN Physical Layer（0727）.doc

Understanding the Physical Layer Perspective of LIN By Frank Kolanko, ON Semiconductor Principal Applications EngineerABSTRACTCost-effective electronic serial communication functionality in non-critical automotive applications are addressed by the LIN (Local Interconnect Network) protocol. As one of a number of emerging industry protocols, LIN offers automotive system designers an important set of conventions to assist them in a highly competitive market.This article delves into the physical layer, the latest implementation options and other fundamental design attributes which need to be addressed when designing modules that utilize the LIN protocol. Topics include EMC, node capacitance, wire length, voltage requirements (positive and negative), history and similarities to ISO9141and integration of additional features. Other topics addressed include module design, master vs. slave nodes, communication techniques and protocol basics. LIN BASICSOne premise must be agreed upon at the onset namely that the primary benefit of using networked communications is that when heavy bundles of bulky wires are replaced with a single (or in some cases dual) wire the primary results are weight savings, improved reliability and the ability to integrate additional features. ISO9141 has long been a preferred protocol used by automotive repair shops in their scan tools to retrieve troubleshooting codes such as OBDII (On-Board Diagnostics). This was the only application for this simple protocol, and is only one of many applications for the LIN protocol. The LIN protocol moves the communication from outside the vehicle to internal communication as well.As with any communication system, the LIN system requires a master and slave device. The basic difference is the impedance with which the bus node is driven. The slave node typically has a 30K impedance. The master node typically has a 1K impedance. Note the required use of diodes in series with the impedance for isolation of unpowered nodes. These may be external to the LIN transceiver IC, or supplied internally by the IC manufacturer. Unpowered nodes are a likely occurrence as system designs focus on low quiescent current. A typical reverse battery diode is also shown in the diagram. Again, this may be supplied internal to the IC. Check your manufacturer specs for maximum reverse voltage on the supply pin. If it is -0.3V, you will require this diode.RESTRICTIONSThe LIN protocol is being developed for non-critical applications within the automobile, but dont let that mislead you into assuming there are no restrictions. EMC is the biggest restriction or problem that needs to be addressed. Each automotive manufacturer has its own requirements, but IBEE Zwickau (University) has become the defacto standard for testing transceivers for all protocols. Among the test results you can expect is the typical bode plot (shown below) for the BUS line response with and without the 220pF (LIN standard) bus capacitance. The university also offers testing for immunity against transients and ESD testing.The bus response highlighted is controlled by the transceiver IC characteristics - which have been designed for good EMC. Slew rate control and wave shaping help to keep high frequencies from being emitted to the environment. Having slew control sets the maximum speed of the bus. Typical transceiver slew rates are 2V/us. The LIN maximum bus speed is 20kbd (50us period). Typical transceiver slew rates are 2V/us. With a 14V battery, this equates to 7us rise and fall times. IC manufacturers then have 36us50us (2)(7us) to waveshape the signal for optimum EMC performance. The minimum limit of the bus is controlled by the RC time constant.Total system node capacitance is recommended by the LIN standard to be typically 4nF with a maximum of 10nF. This includes intended node terminations, extraneous cable capacitance, and transceiver output capacitance. This will provide the best EMC / EMI performance. This should be adjusted at the master node, using the 220pF capacitors on the slave nodes. This adjustment is typically needed for small (number of nodes) networks.Slave node termination RESISTANCE is the second part of the equation. This is the parameter which must be tweaked for large (number of node) networks as the system design will reach the maximum capacitance as defined by the LIN spec. The LIN specification we must pay attention to when tweaking the resistance is the system time constant of 1.0 us (min) and 5.0us (max). The recommended Slave Terminal resistance is 30K. You must worst case your system pushing all parameters to the condition which creates the worst time constant. The recommended wire should have a characteristic of 100 pF/m with a limit of 150 pf/m. The LIN specification also calls out a limit of 40 meters for your LIN bus. The 40 meter limit is not really the important spec. Rather, when the 40 meter limit is combined with the 150pF/m spec, this yields a 6nF maximum limit for the wire alone. This leaves you with 4 nF (you would think with the10nF total bus capacitance spec) for you to customize all your bus nodes. But if you are trying to center your spec (as you should be) around the tau spec, you will find you will not be able to because you would violate minimum total resistance spec.SYSTEM DESIGNHere is a chart of the parameters you have control over. When designing the system, first focus on the line capacitance. As you can see it plays a critical part in the design.LIN Physical Layer SpecificationsMinMaxTau spec1us5 usTotal resistance for a 16 node (maximal) network537W863WTotal Capacitance1 nF10nFAlso note the percentage variation from the mathematical center for the capacitance is greater than the mathematical center for the resistance. Additional testing results (from IBEE Zwickau (University) for pin response with and without frequency disturbance. IMPLEMENTATION OPTIONSNow that you know something about the system, lets see if it will fit your application. First and foremost, LIN is not intended to be used in any safety systems. LIN is intended for non-critical applications. How many nodes does your system require? LINs limit is 16 nodes. Systems with greater than 16 nodes will load the system down given the limitations dictated by the 1K master node impedance in parallel with each slave nodes 30K impedance. Running additional nodes risks fault free communication.There are a few options to choose from when considering a LIN transceiver. One option is shown below where the transceiver is on all the time there is a voltage supplied to the module. Low quiescent current LIN transceiver devices are available for this type of application.If your system has lower quiescent current requirements, LIN transceivers with sleep modes will work. Quiescent current can be kept to a minimum by waking the LIN transceiver only when needed via a signal on the LIN bus, and correspondingly having the LIN transceiver wake up the module regulator.IC manufacturers have also taken the opportunity to combine the voltage regulator with the LIN transceiver (see diagram below). Note as with any IC the absolute maximum junction temperature must not be exceeded. Using a linear regulator topology makes the IC susceptible to increased die temperatures. Operating at extended levels c