In any vehicle, the instrument cluster forms a critical part as it is the face of the vehicle that reflects the current state. Apart from providing the basic vehicle information, it can also provide crucial indications of electrical component malfunctions (EFI /ISG related). Clusters can acquire vehicle data over simple mechanisms such as encoders as well as through complex interfaces such as CAN, SAEJ1850 etc.,

Instrument cluster design for electric vehicles may have less system complexity, but they have to be more user friendly, less power hungry, and a cost-effective solution. Careful design and choice of cluster components is necessary to meet the stringent compliance requirements. Of the constituent components, the MCU holds the major stock whereas only a few vendors will be supporting automotive grade with the desired feature sets.

Renesas RL78 is an ultra-low power microcontroller (MCU) that enables customers to build compact, energy efficient instrument clusters for a wide range of two-wheeler and four-wheeler vehicles. It offers highly suitable peripheral interfaces such as: Three Channels of Stepper Motor Controller, LCD controller, Multiple IOs etc.,

In this blog, we will discuss in detail about the various features of an instrument cluster and how RL78 MCU will help simplify the design.

Instrument Cluster Subsystem

The major components and their functionalities are as follows:

• Gauges: Generally, three gauges namely RPM, Temperature and Fuel Level get indicated by the cluster.
  • Fuel Gauge: The Fuel gauge ranges from Empty (E) to Full (F). Typically, the Fuel sensor unit is a TSR /SMR type realized with components such as float with potentiometer, Adjustable arm and sending unit with two wires (signal & ground wire). The signal received from the fuel sensor is analog. The pointer position depends upon the incoming resistance value and indicates the vehicle’s fuel level.
  • RPM Gauge: The signal received from the RPM sensor from encoder such as tach terminal unit with signal & ground wire, fire wall grommets is treated as frequency input. Based on the type and top speed of the vehicle, the input range varies.
  • Temperature Gauge: The signal received from the temperature sensor is also analog. Realized with sensors such as protective rubber unit, thermistor with two wire. Front end circuit converts the input resistance to corresponding voltage and thereby the voltage value at the microcontroller input indicates the current engine temperature.
  • Seven Segment Displays: Clusters may have different type of segmented display with various resolutions such as 27 X 4, 39 X 4, 48 X 4, 53 X 4 and 54 X 4. These could indicate information such as odometer, trip distance, hour meter etc.,
  • Telltales: Telltales are sort of regulatory indications typically realized with LEDS required to show the state of various components such as low battery, engine faults, high beam etc. They are usually realized via onboard LEDs with mechanical light guide. Graphical overlays will also be placed over the guides with standard Tell tale signs.
  • Switches: Usually a single switch or simple rotatable push-type switches are available to manipulate the information shown in the display such as Trip distance, Time etc.

There could be variations in the offerings and feature set based on the vehicle manufacturer.

RENESAS RL78 for Cluster:

Renesas is a leading semi-conductor company in the automotive space offering cluster solutions for many decades. While there are some architectural differences between all the MCU’s of the Renesas RL78 family, the sub-system remains more or less the same. The high-level design of the RL78 MCU based instrument cluster is captured below:

RL78 series of MCU’s help realize vehicle clusters effectively with its feature rich peripherals set:

Microcontroller: The RL78 MCU helps in achieving the highest performance within the low end MCU space. It offers improved efficiency, extensive scalability and reliable safety functions that aid in developing energy efficient instrument clusters. It has an operating voltage that ranges from 2.7V to 5.5V and flashes the program using 80 microcontroller pins.

Stepper Motor Drivers: There are up to three channels of stepper motor controllers / drivers with zero-point detection that can be configured using PORT pins (Sin+, Sin-, Cos+, Cos-). Each channel is controlling a stepper motor dedicated to gauges which commands the motor to move and hold at one of the angles without any position sensor. With the help of software based PID control, it is possible to effectively control the needles.

Segment LCD Controller: Capable of driving up to 200+ segments, the in-built LCD controller is available for controlling the segmented display with various resolutions such as 27 X 4, 39 X 4, 48 X 4, 53 X 4 and 54 X 4 which is directly driven from the RL78 microcontroller. The LCD screen can be controlled by the software to display any kind of data dynamically such as TRPI information, hour meter data etc.

Multiple IOs: There are multiple analogs and digital IOs available for interfacing sensors and Tell-Tale LEDs. RPM, temperature, and fuel level sensors are interfaced via a suitable front-end circuit. If needed, various Tell-Tale LEDs such as turn left, turn right, service, brake, etc. can be driven with dedicated digital outputs.

• Analog Input Gauges: RL78 series comes with a rich set of ADC inputs that can be used to convert resistance/voltage-based sensor inputs to digital values. Software based processing will increase the accuracy of the acquired value.
• Encoder inputs: With up to 24 timers, RL78 can be used to measure a variety of encoder inputs such as RPM, perform custom processing and obtain accurate digital values.
• External connectivity: RL78 can also support I2C/SPI interfaces that can be used to communicate with peripherals such as EEPROM where the accumulated vehicle information such as ODO, TRIP data can be stored. RTC can be interfaces to track real time.  

Variants of RL78 also offers sound generator which can set a volume level and tone frequency, two channels of CAN, and expanded flash memory/RAM up to a maximum of 512 KB/24 KB, suitable for low-end instrument cluster applications.

Embien’s Expertise in the Automotive Industry:

Embien Technologies is a leading embedded systems development company that specializes in high – tech product engineering services. We help clients take competitive lead in their markets. With a unique mix of innovation, technology, and quality, we offer cutting-edge automotive electronics system designs by adapting the latest technologies in clusters, diagnostics, infotainment, connectivity etc., With deep domain expertise, we have assisted several OEMs, Tier 1, and Tier 2 vendors to surge their automotive electronics such as Instrument Clusters (Heavy & Light- Duty Vehicles), Digital Cockpits etc., by integrating sensors & radars, network connectivity, path planning & mapping technologies. Do you have a design idea in mind? Then, get in touch with us today!