Issue 150 January 2003
Going Mobile

FRAME UP

With the previous explanation in mind, understanding the LIN message structure is a snap (see Figure 2). The action starts when the master issues a sync break, driving the bus low for 13 bit times. This is the single area where LIN is a little fussy about UARTs. Those who don’t have hardware break generation and detection will have to use an interrupt and/or I/O line. Of course, the same goes for bit-banged implementations, which are feasible thanks to the relatively low speed and complexity involved.

Following the break, the master issues a sync byte, 0x55. Slaves use the alternating ones and zeroes to align with the master’s time base. This is a key feature that allows LIN devices to get by with relatively sloppy (i.e., ±15%) on-board (typically RC) clocks. Of course, if both the master and slave utilize accurate clocks (e.g., crystal), the entire issue can be ignored.

Finally, the master issues the aforementioned identifier byte comprised of a 4-bit ID code, 2-bit length field, and even and odd parity bits. At this point, all slaves have detected the sync break and established proper timing. After receiving the identifier byte, the appropriate slave responds with 2, 4, or 8 bytes of data (depending on the length field in the identifier byte) and a checksum. Thankfully, there are no rigid response time specs, making life easy for everyone. The only restriction is that the overall response must arrive within the maximum time allowed for a single message in order to allow a loss of communication to be detected.

Of the 64 unique identifiers that are available (i.e., combined 4-bit ID and 2-bit length), four are reserved by LIN for administration (e.g., upload and download firmware) and future extensions. One of the administrative commands the master can issue is Sleep, which tells all of the nodes that they can ignore the LIN bus until further notice.

Said notice comes in the form of a bus wake-up signal, which is like a Sync Break (nominally eight bit times versus the latter’s 13), except that the slave issues it. The wake-up signal tells the master to start polling and alerts other slaves to get ready for LIN bus action (i.e., the forthcoming sync break issued by the newly woken master).

Yes, the master doesn’t know which slave issued the wake-up call, and the service time will be determined by that slave’s place in the polling pecking order; however, at least that service time is completely predictable because, like all other LIN timing, it’s totally under the master’s control.

The bottom line is that every time you’re about to raise an objection and say, "But…," just remember we’re talking about switches, lights, motors, and humans (i.e., a timescale that even the slowest incarnations of LIN can manage).