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RAMBLINę MAN

by Tom Cantrell

Start ę Little Network ę Mini-Message ę LO BAT ę Caravan ę Sources and PDF

LITTLE NETWORK

Actually, LIN stands for Local Interconnect Network, not little network, but you get the idea. Itęs an econobox Class A network for all those proliferating lights, switches, and motors at a low cost that CAN canęt match.

LIN is the proposal of a largely European consortium of automakers (Audi, BMW, Volvo, and VW) with some U.S. participation from recently married DaimlerChrysler and U.S. automotive-electronics kingpin Motorola. The specs and first generation tools were originated by the final consortium member, Volcano Communications Technologies AB.

LIN incorporates many design features and eschews others, in a manner unique to the task at hand. Itęs a simple UART-based (ISO 9141 NRZ, in auto spec terms) scheme that sacrifices speed and fancy desktop network wannabe pretensions in favor of simplicity and low cost.

Electrically, LIN is a single-wire (up to 40 m), wired-AND arrangement (like I²C), where each node is connected by a pull-up resistor to the bus and any node can transmit by pulling the line low (see Figure 2).

Figure 2ęThe basic wired-AND configuration is similar to I2C, but LIN is even simpler. (enlarge)

The LIN bus runs off the car battery (12 V), thus, eliminating many electrical and wiring hassles right up front. The downside is that the bus needs to handle power surges up to 40 V, polarity reversal, and such, but that dirty work is handled by off- and on-chip transceivers.

The single-wire configuration and large voltage swing dictate a limited slew rate (typically 2 V/ęs) to minimize EMI concerns. That, in turn, limits the maximum data rate to 20 kbps, which is just as well. Itęs fast enough to handle lights-and-switches duty, but unlike J1850/CAN, can easily be bit-banged by any 8-bit micro.

Itęs more than the speed limit that differentiates LIN. After all, itęs roughly similar to current J1850 nets. Rather than raw bandwidth, itęs the way the LIN bandwidth is used that sets it apart.

The major distinction is that LIN is a polled, rather than a contention, network. In LIN, slaves speak only when spoken to. Yes, LIN does require a master node to run things (i.e., poll), which represents a potential single point of failure. However, thatęs also true of many other electrical components in modern cars, and silicon is more reliable than most. If thatęs not good enough, a combination of explicit design redundancy and limp-home backup can be applied.

In recompense, the master-slave approach has some practical benefits. The master controls all communication. There is no arbitration or automatic retry. The message duration is either bound by the LIN specification or under the softwareęs control. Put it all together and itęs possible to achieve accurate timing and guarantee the worst-case service interval for each node.

In fact, what timing uncertainty LIN does have is largely a by-product of the data rate being allowed to drift quite a bit (ę15%). Indeed, all nodes re-synchronize their timing to the master on every message. The benefit is that, unlike the typical fixed data rate setup, the nodes donęt need expensive, fragile crystals, but can instead use simple (albeit sloppy) RC clocks.

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