The paramount concern in automotive design is safety, tasking designers with adding more sensors and driver assistance systems. The problem is that these systems need a common interface and must be able to communicate reliably and error free despite wide temperature and humidity swings, vibration, and electromagnetic compatibility issues.
For designers, it’s worth taking a look at sensor-based solutions compliant with the peripheral sensor interface (PSI5). This is a rugged, interference tolerant interface originally used for airbag systems, but which is increasingly finding newer sensor intensive automotive applications.
This article will provide an introduction to the PSI5 bus before introducing and describing a number of PSI5 system solutions and how to use them to configure a sensor-based control system.
The peripheral sensor interface (PSI5)
The PSI5 interface is used to connect multiple sensors to electronic control units (ECUs) and has been used as the primary sensor communications bus for airbags and related restraint systems. It is an open standard available from the website of the PSI5 organization at PSI5.org. The current specification is the PSI5 version 2.3, released as a base standard common to all sub-standards including those for airbag, chassis and safety control, and powertrain.
The PSI5 standard is implemented as a two wire (twisted pair) bus using current modulated, Manchester encoded data transmission at data rates of 125 kbps (189 kbps is optional). It is a medium speed interface compared to the other common automotive data buses (Table 1).
Interface bus Physical connection Max bit rate Max length at max bit rate
LIN 3 wire 19.2 kbps 40 m
PSI5 2 wire 189 kbps 12 M
SENT 3 wire 333 kbps 5 M
CAN 4 wire 1 Mbps 40 M
FlexRay 2 or 4 wire 10 Mbps 22 M
Table 1: Comparison of common automotive data buses. PSI5 is a medium speed interface. (Data source: Digi-Key Electronics)
PSI5 holds an advantage in the middle speed range being less expensive to use than CAN or FlexRay, but having a data rate compatible with sensor data. The SENT data bus is also intended for sensor data transmission, but is limited by only being able to send data from the sensor to the electronic control unit (ECU). PSI5 is bidirectional, allowing for the addressing and configuration of sensors.
A typical implementation of PSI5 in an automotive ECU includes a microcontroller feeding multiple interfaces (Figure 1).
Block diagram of an automotive microcontroller ECU
Figure 1: A block diagram of an automotive microcontroller ECU, which includes I/O ports for common automotive data buses, including PSI5. (Image source: Digi-Key Electronics).
The boxes to the right of the microcontroller in Figure 1 show the supported I/O ports. These include Ethernet, Controller Area Network (CAN), Local Interconnect Network (LIN), and FlexRay automotive communications buses, along with Single Edge Nibble Transmission (SENT) and PSI5 sensor interfaces. These automotive ECUs are highly integrated and may include delta-sigma analog-to-digital-converters (ADCs) for fast and accurate sensor measurements.
PSI5 physical layer
The ECU is connected to the sensors using two wires. The use of a two wire twisted pair reduces the cost of implementation compared to the other buses which use three or more wires. The same two wires are used for power and data transmission. The ECU may use an integrated or a separate PSI5 transceiver to provide a regulated voltage to the sensors and to read their transmitted data. Sensor data is transmitted to the ECU by the means of current modulation using Manchester encoding (Figure 2).