Issue 150 January 2003
Going Mobile

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CHIPS AND DIPS

Witness to the credibility of LIN is the off-the-shelf availability of a spectrum of silicon solutions from a number of major players. Presuming you’ve already got a UART (hardware or bit-banged) and a handle on the protocol, getting on the LIN bus is as easy as adding a transceiver such as the Philips TJA1020 (see Figure 3).

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Figure 3—Turning a UART into a LIN port is as simple as adding the protocol software (typically 0.5 KB of code or so) and a LIN transceiver, such as the TJA1020 from Philips Semiconductor.

There are only eight pins to deal with, but appearances can be misleading. Just standing up to the challenge of an automotive electrical environment is tough. Direct battery connection calls for all manner of protection against transients, short-circuits, loss of power, and so on.

The Philips transceiver goes further with extras that simplify the overall design. For instance, a node that’s sleeping can be awakened remotely (i.e., via a wake-up message on the LIN bus) or locally (NWAKE pin), and the transceiver informs the MCU of the wake-up source.

Both the transmitter and receiver feature line-monitoring sanity checks. If the LIN bus is shorted to ground, the receiver detects it and puts the transceiver to sleep, shutting off current flow that might otherwise drain the battery. Similarly, if the transmitter input TXD is continuously asserted, the transmitter is automatically shut down so that one jabbering node doesn’t bring the entire network down.

The INH pin is an output that’s driven with the battery voltage when the transceiver is awake. Thus, it can be used to cycle power to the node (i.e., MCU), either by controlling or, with up to 50 mA on tap, supplying an external voltage regulator.

Going one step further, the Melexis TH8060 combines the LIN transceiver with an on-chip regulator (5-V, 50-mA, or 100-mA versions) and RESET output. The chip also includes low-voltage detection (SENSE) and a general-purpose analog comparator (SI and SO). The TH8060 connects easily to your favorite MCU and automatically handles power-on and reset via LIN bus wake-up.

The popularity of LIN is further highlighted by the emergence of MCUs with built-in transceivers (e.g., the Microchip PIC16C432 in Photo 2) and other LIN-oriented support, such as the 2% trim on-chip oscillator and LIN-savvy break detection logic on the Motorola MC68HC908EY.

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Photo 2—The PICDEM LIN evaluation board from Microchip shows off the new PIC16C432 and ’433 MCUs with built-in LIN transceivers.

Motorola offers yet another variation on the theme with so-called "system basis chips" that throw a LIN transceiver into a comprehensive bag of peripheral tricks. Examples include the MC33689 and MC33895, which are designed for applications using DC motors, stepper motors, lamps, and so on (see Figure 4). With built-in LIN transceivers, these chips will help seed standards in automotive applications and beyond.

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Figure 4—LIN is just part of the package for the MC33895 chip that combines the network transceiver with a variety of blue-collar I/O functions.

The Cypress PSoC is a novel chip that, not surprisingly, has a novel take on the LIN subject. Messages are handled by three separate programmable logic configurations that are dynamically swapped in real time, cutting the amount of logic consumed by two-thirds (see Figure 5).

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Figure 5—Dynamic reconfiguration comes out of the lab and goes to work in the Cypress Micro Systems PSoC implementation of LIN.

LIN SPIN

There’s no doubt LIN is destined for success in automotive applications. There’s also no doubt that relatively few embedded designers work for car companies. So why bother?

The fact is that LIN, like CAN before it, is likely to evolve into other application areas. For instance, Motorola notes that the aforementioned MC33xxx chips are not only ideal for a car door or seat, but just as well for copiers, printers, robotics, computer numerical control machining systems, and electrical actuators in appliances.

Microchip specifically mentions the security and fire protection market, making a persuasive argument that LIN is ideal for white goods or appliance applications. [2]

Yes, there are a lot of contenders for such LAN-in-a-box applications, but, whether you consider it a kind of CAN-Lite or I2C on steroids, it’s clear that hardiness, availability, cost, and simplicity stand in favor of LINs.

One thing’s for sure: with close to 10 million 8-bit MCUs shipping every day, the party has just begun. Whether a car or a washing machine, pop the hood on the latest model, and you’re going to find ever more micros and ever more connections between them.