S
CANH
CANL
FAULT
TXD
GND
V
CC
RXD
FAULT
DTO
DTO
Introduction
1
Introduction
1.1
Overview
Texas Instruments offers a broad portfolio of High Speed (HS) CAN transceivers compatible with the
ISO11898-2 and ISO11898-5 High Speed CAN standards. These include 5V V
CC
only, 3.3V V
CC
only, 5V
V
CC
with IO level shifting and galvanic isolated CAN transceivers. These CAN transceiver families include
product mixes with varying features such as low power standby modes with and without wake up, silent
modes, loop back and diagnostic modes.
The Texas Instruments SN65HVD257 CAN EVM helps designers evaluate the operation and performance
of the SN65HVD257 CAN transceiver. The SN65HVD257 includes many features for functional safety
network implementation such as redundant CAN networks. The SN65HVD257 CAN EVM also provides
PCB footprints for different bus terminations, bus filtering, and protection concepts. The EVM is provided
with two SN65HVD257 devices installed. A separate EVM is available for the other CAN transceivers,
SN65HVD255 CAN EVM, and another EVM uses the galvanic isolated CAN transceiver family (ISO1050).
The SN65HVD257 meets the ISO1189-2 High Speed CAN (Controller Area Network) Physical Layer
standard (transceiver). It is designed as a next-generation CAN for the SN65HVD251 and ISO1050, but
with added features for functional safety networks such as redundant networks. It has very fast loop times
with a wide range of bus loading, allowing for data rates up to 1 megabit per second (Mbps) in long and
highly loaded networks and higher data rates in small networks. The device includes many protection
features to provide device and CAN network robustness. The device has two modes: normal mode and
silent mode, selected on pin 8. The FAULT pin indicates TXD dominant time out, RXD dominant time out,
thermal shut down and under voltage faults.
Figure 1. SN65HVD257 Basic Block Diagram and Pin Out
1.2
Example Using the SN65HVD257 in a Redundant Physical Layer CAN Network
Topology
CAN is designed for standard linear bus topology using 120
Ω
twisted pair cabling. The SN65HVD257
CAN device includes several features that allow use of the CAN physical layer in nonstandard topologies
with only one CAN link layer controller (
μ
P) interface. The SN65HVD257 allows much greater flexibility in
the physical topology of the bus while reducing the digital controller and software costs. The combination
of RXD dominant time out and the FAULT output provides great flexibility, control and monitoring of these
applications.
A simple example of this flexibility is to use two SN65HVD257 devices combined logically in parallel via an
AND gate to build a redundant (parallel) physical layer (cabling and transceivers) CAN network. Adding a
logic XOR with a filter adds automatic detection for a fault where one of the 2 networks goes open
(recessive) in addition to the faults detected by the SN65HVD257.
To allow CAN’s bit-wise arbitration to work, the RXD outputs of the transceivers must be connected via
AND gate logic so that the link layer logic (
μ
P) receives a dominant bit (low) from any of the branches; the
transceivers appear to the link layer and above as a single physical network. The RXD dominant time out
(DTO) feature prevents a bus stuck dominant fault in a single branch from taking down the entire network
by returning the RXD pin for the transceivers on the branch with the fault to the recessive state (high) after
2
SN65HVD257 CAN EVM: Functional Safety and Redundant CAN Network
SLLU172 – August 2012
Copyright © 2012, Texas Instruments Incorporated
