Ten things to look for when choosing MCUs for automotive designs

Waqar Saleem, Fujitsu Semiconductor

December 03, 2012

Waqar Saleem, Fujitsu SemiconductorDecember 03, 2012

Microcontrollers (MCUs) deliver vital performance in an increasingly wide range of automotive applications, from motor control to infotainment systems and auto body control. Their popularity is growing at the same time that price sensitivity and consolidation are increasing, meaning that MCUs are viewed more and more as commodities. Despite this commoditization trend, designers of auto systems understand that there are significant differences between controllers, including varying levels of integration and power requirements. The choice of an MCU can often reduce the Bill of Materials (BOM) cost, which effectively lowers the price of the electronic control unit (ECU) itself.

When selecting an automotive MCU, designers can balance the cost pressure with the specific performance features needed for their applications by considering 10 major factors.

1. Low voltage detection
One of the failure risks during MCU operation is that the power supply voltage or the internal MCU voltages may drop below the required level at a critical point. Obviously, this can cause a malfunction, as the operation is not guaranteed outside the recommended voltage supply.

Traditional systems used an external voltage-monitoring IC to check the voltage. However, it is possible to integrate that function into the MCU with an internal block that monitors both the internal MCU voltage and the external voltage supply levels. If the level falls below a preset threshold value, the MCU is reset automatically, as shown in Figure 1. The threshold level can be selected from a set of seven pre-assigned values, as in case of the latest Fujitsu MCU. Such an approach eliminates an external component from the BOM, reducing costs.



Figure 1: Low Voltage Detection and Automatic Reset

2. Watchdog timers
Another key feature to look for is the watchdog timer, which helps recover from failure situations such as a “runaway micro” or “processor in the weeds.” The module resets the MCU as soon as it detects that the MCU is unresponsive. In the past, embedded systems used an external IC to perform this function. However, multiple watchdog timers can be built into the MCU. For example, one timer could operate as an independent clock outside of the CPU operation system clock. This timer, which would be based on a slower CR clock, would be suitable for use as a hardware watchdog for the MCU, or for use by longer software loops to prevent runaway conditions. The other timer could operate based on a faster peripheral clock. Ideally, the watchdog timer modules would support the window function that also resets the MCU when the timer is fed too quickly, which probably would be due to some erroneous condition.



Figure 2 - Built-in Watchdog Timers

3. Dedicated NV memory
Like the watchdog timers, EEPROMs have traditionally been external components to MCUs. However, it is possible to make such memory storage an internal component by using dedicated ROMs. The built-in EEPROM can be further enhanced by increasing endurance and using error-correction mechanisms.

A sophisticated approach to incorporating EEPROM internally is to use dual-operation Flash. One part of the Flash memory bank can be read while the other bank is being programmed, allowing the EEPROM functionality by a single Flash module. The other style is to implement two Flash modules, but this can result in larger overhead than dual-operation Flash. The Fujitsu MCUs, for example, have a high-endurance EEPROM implementation with a rating of 100,000 erase/program cycles. The MCUs also support ECC and can retain data for 20 years. Commercial-grade software is available to control Flash memory as the EEPROM function.

4. Automotive grounding
Electrical connections in an automotive environment can be physically quite long because of the way the electronic control units are positioned. Automotive systems contain a number of ECUs and other equipment drawing relatively large current. As a result, the electrical ground level is not perfect and can float within a certain range, in addition to the parasitic noise generated by the ECUs themselves.



Figure 3: Floating Automotive Ground

Designing the MCU according to such ground conditions can increase the robustness and safety levels against failures. Sophisticated MCUs are designed for standardized VIL according to automotive conditions. That helps prevent errors that could occur because of the “floating ground,” improving the quality of the ECU.

5. Vbat level direct input
Some ECUs in automotive systems operate the IO signals around the battery-level voltage. For semiconductors designed based on CMOS technology, the IO signals are VCC level maximum, usually in the 3V to 5V range. Hence, transceivers are needed for voltage-level translation. In some cases, it might be possible to build in voltage protection, which would allow the high voltage signals to be connected directly through a current-limiting resistor. The Fujitsu MCUs are designed to support such usage by an internal protection diode with an external current limiter. Such an approach reduces the number of components needed on the PCB, again lowering costs.



Figure 4: Directly Input Vbat Level Signal

6. Terminal function relocation
Maintaining the lowest possible layer count when doing a PCB layout for an IC with a significant pin count is often challenging. The peripheral components on the PCB cannot always be ideally located according to the pin-out of the MCU. Sometimes it would help if the MCU had the built-in flexibility to relocate its internal modules to a different set of pins. This could be done by a software setting. This capability can increase flexibility during the PCB layout process.



Figure 5: IO Terminal Relocation

7. ADC-assist functions
Analog to Digital Converters (ADCs) have been a fundamental block of embedded systems for a long time. ADCs convert signals from the analog to digital domain, enabling access to information coming from the analog world.

It may be possible to distinguish the MCU on the basis of the ADC block by adapting the block to specific applications. The enhancements could distinguish the whole MCU package. For example, the ADC module could support the range-comparator and pulse-detection functions in hardware. These are useful for applications such as stepper motor control in instrument clusters, power monitoring, and sensor applications. The ADC could process the input signals from the stepper motor coils to execute Zero Point Detection (ZPD). With the processing done in hardware, the CPU can use its MIPS elsewhere.

8. LIN hardware-assist functions
Local Interconnect Network (LIN) is an inexpensive, low-speed communication technology that is used extensively for body applications. By implementing functions such as automatic header transmission and detection, communication test function, variable break length generation, and checksum generation and verification in hardware, MCU performance with LIN can be enhanced. That approach helps save the CPU’s MIPS for use elsewhere.

9. ZPD enhancements
For instrument cluster applications, the ECU uses Zero Point Detection to determine when a needle has reached the end point so that the stepper motor can be stopped. This function requires that the stepper motor controller (SMC) read and evaluate the voltage signal (also called the “back EMF”) in the motor coil to make the detection. The SMC can be enhanced by adding hardware support for voltage evaluation, so that no external components are needed to implement ZPD. Also, most of the back-EMF evaluation can be done using a hardware mechanism. (In that regard, the ADC range comparator and pulse-detection functions mentioned earlier are helpful.) Again, this approach requires minimal CPU use.

10. Position and revolution counter
It is advantageous to have the Quad Position and Revolution Counter (QPRC) functionality available in the form of a hardware block. This allows users to implement jog-dial functions for audio and navigation applications. The module can control the rotation extent and direction, and determine the rotation speed. Theoretically, this can be done using the standard input capture unit in the MCUs. However, having a specialized hardware module for these tasks allows the CPU to conserve resources. The result is a better task allocation within the system, and a simplified software package.

Selecting the Optimal Supplier
There is one more critical factor to consider when selecting an automotive MCU: look for a company with a long history and a significant market share. Also consider whether the supplier offers a wide range of MCUs for a broad spectrum of automotive applications, including body, power-train, and driver-information systems. Look for an automotive product line that includes 16- and 32-bit MCUs based on industry-proven, proprietary CPU and standard ARM architectures.

In summary, despite the trend toward the commoditization of automotive microcontrollers, MCUs provide different special functions that can enhance system performance while not necessarily increasing costs. Selecting automotive MCUs carefully can significantly increase the potential for cost-effectively differentiating the final product. And selecting an MCU vendor with a solid reputation, wide range of offerings, and strong support will make the automotive MCU design process easier and more efficient.

Waqar Saleem is a senior applications engineer with Fujitsu Semiconductor America, based in Detroit, Michigan. He has more than a dozen years of design and applications experience, and holds engineering degrees from San Jose State University and the University of Engineering and Technology in Lahore, Pakistan.

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