Servo - June 2009

Implementing a Low Speed, Low Cost

Communications Protocol

by Fred Eady

(that you’ve probably never heard of)

The LIN (Local Interconnect Network) framework was originally conceived for the automobile industry. LIN nodes are typically small standardized electronic modules that enable low cost networks for low end automotive functions. In today’s automobile, major automobile computing nodes are usually interconnected using high bandwidth CAN (Controller Area Network) technology. LIN nodes command less bandwidth than CAN nodes and are normally found in smaller satellite networks that oversee the automobile’s door locks, windows, lighting, and air handling. If you really get down to it, a modern automobile is made up of a number of networked embedded computing nodes that are tasked to support the automobile’s mechanical parts and electrical functions. Thus, a car is no more than a sophisticated, computerized four-wheeled cargo robot.

LIN is a relatively young network topology. It is a standardized protocol based on hardware and firmware normally used for RS-232 communications. Initially released in 1999, the latest spec is revision 2.0 and dated September 23, 2003. As LIN became more of a complicated liability for American automakers, North American automakers dived into the LIN mix. A task force was assembled and published the SAE (Society of Automotive Engineers) J2602 Recommended Practice for LIN Networks document, which is aimed at the ambiguities of the LIN 2.0 specification. As of late 2008, a LIN 2.1 test specification is in the works. If LIN can be applied to the control of automobile mechatronics, it stands to reason that we can adapt LIN to our robotic needs, as well. Let’s begin our smells-more-like-a-robot-than-a-car LIN discussion by

PHOTO 1. The LIN node electronics are mounted on an inexpensive two-layer ExpressPCB printed circuit board. All of the components are readily available from Mouser or Digi-Key. There’s plenty of breadboard area for mounting relays, LEDs, an LCD, or motor drivers.

36 SERVO 06.2009

designing and building a pair of regulation LIN nodes.

Scratching Out A Robotic LIN Node

From the looks of Photo 1, we’ve got some assembly work to do. The actual LIN circuitry takes up about half of the printed circuit board’s component area. The rest of the component area is populated by a textbook PIC18F2620 microcontroller implementation. I’ve reserved plenty of breadboard area for additional monitor and control electronics as the automotive and robotic purpose of a LIN node is to perform or control a mechatronic task.

The Microchip LIN transceiver doesn’t need much support from the PIC18F2620. So, most of the PIC’s I/O pins are available at the I/O header. If you need to commandeer some of the I/O pins that are already spoken for, you can free up RC0 and RB2 by removing resistors R9 and R8, respectively. By activating the PIC’s internal oscillator, you can also recover I/O pins RA6 and RA7 while eliminating crystal Y1 and supporting capacitors C8 and C9. The LIN founders intended for the slave nodes to be hosted by devices that require very little support circuitry, such as crystals and voltage regulators. So, you may want to keep the crystal and its supporting capacitors in your master node and trim the slave nodes down to the bare essentials.

There are two things that turn our PIC18F2620 circuit into a LIN node: the firmware driver and a Microchip MCP2021 LIN transceiver. Since we’re in the hardware build mode, let’s talk about the MCP2021.