The serial peripheral interface (SPI) and universal asynchronous receiver/transmitter (UART) have become the popular communications protocols for microcontroller-based industrial systems. For embedded systems that require greater robustness and connectivity, controller-area network (CAN) and Ethernet are the first choices for many developers.
While all of these protocols support transfer speeds greater than 1 Mbit/s, they’re still used in lower-bandwidth applications because they’re well understood and supported.
Local interconnect network (LIN) and inter-integrated circuit (SMBus/I2C) are two viable alternatives for non-bandwidth-intensive applications. These two protocols offer a number of benefits for embedded systems, and they often become an ideal choice relative to other protocol options for industrial applications.
LIN Protocol
Historically, LIN has been used in automotive systems. But its advantages such as low-pin count, data integrity, easy implementation, and standardized application interface fit many embedded applications.
The LIN Consortium created the LIN bus standard to provide a common communications platform for vehicles. Some of the primary design constraints for the protocol included low cost, high reliability for data transfer, and the high reliability for the hardware itself due to a demanding operating environment.
A LIN network exhibits the simplicity and low-cost development benefits of the UART and SPI protocols, but offers data reliability and some scalability provided by CAN and Ethernet. To understand how this network meets these design goals and offers other benefits, it’s helpful to examine the standard network architecture.
LIN networks (Fig. 1) contain a single master node and up to 16 slave nodes. At the system level, the master nodes are typically powerful MCUs, and the slave nodes are simpler MCUs or ASICs. The LIN network is a serial broadcast network that supports baud rates up to 20 kbits/s. Nodes connect to the same single-wire bus via transceivers.
At the physical layer, the master initiates all data transfers, which are then transferred in frames measuring up to 8 bytes in length. Each frame includes a synchronization break field, a synchronization byte, an identifier byte, data bytes, and a checksum byte.
Different types of frames will transfer data and send diagnostic messages. Unlike most communication protocol standards, the LIN specification extends to the application level and specifies an application programming interface (API) for software drivers.
Assuming that the system doesn’t require greater than 20 kbits/s of data transfer, these design specifications garner benefits for the embedded system and the system developer. From a cost-per-node perspective, LIN networks are more cost-effective to implement than comparable CAN and Ethernet networks.
Also, LIN transceivers are small, eight-pin devices that are readily available for less than $0.75 in moderate to large quantities. Their low pin count helps reduce board size as well.
Furthermore, LIN transceivers are automotive-quality certified and thus tested to high electrostatic discharge (ESD) tolerance, which can mean fewer replacements coming back from the field. Ethernet transceivers target commercial applications and usually cost less than $2.
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