Selecting a Microcontroller for Handheld Measurement Applications

Selecting the right microcontroller (MCU) for measurement applications has become an increasing challenge as MCUs vary in cost, peripheral design and mix, CPU architecture, and on-board integration. For portable industrial measurement applications, the most important factors are battery life, high-performance analog peripherals, and appropriate user interfaces, which determine which MCU will be the best choice for the measurement application.
Long battery life
To maximize battery life, the design must minimize average current consumption. Designers should not only consider operating parameters, but also calculate the average power consumption under all operating conditions. If the decision is made rashly, the designer may make a power usage mistake that results in excessive power consumption. Today's low-power MCUs offer multiple operating modes such as active mode, standby mode, and power-off mode. In active mode, all clocks are in operation; in standby mode, the CPU clock is inactive, but the real-time clock is running, waiting for an interrupt event to wake up the CPU; usually MCUs have different levels of standby operating modes. In power-off mode, all clocks are inactive, waiting for an interrupt event to wake up the CPU. To understand the power consumption of different operating modes, we must consider characteristics beyond the datasheet. Some datasheets give current consumption under unrealistic operating conditions. To get an accurate picture, we compare the maximum current consumption of different operating modes in the actual application environment at a specific operating voltage and operating temperature, usually 3V and 25oC for lithium-ion batteries.
Another way to extend battery life is to minimize the transition time between standby or power-off mode and active mode. Some manufacturers have introduced "on-demand" clock sources designed to provide a stable clock source immediately after an interrupt assertion.
The MCU can wake up the CPU using flag polling or interrupt vectors. Interrupt-driven architectures can significantly save power because the CPU can respond immediately to any event without wasting current to poll peripherals.
The power consumption of standby mode is lower than that of active mode. The MCU we choose must minimize the standby current of the CPU. MCUs designed for low power consumption can automatically trigger analog-to-digital conversions using timers and data without CPU intervention through direct memory access (DMA) and buffer sampling. When the CPU is idle, it can be used for other work, which helps improve system throughput. In addition, we can turn off the CPU to reduce the average power consumption of the application.
Minimizing leakage current is critical to reducing power consumption. For most portable applications, the end product is not working most of the time, so the leakage current determines the average current consumption. We need to calculate the leakage current of the entire port and ensure that the port configuration can minimize current consumption. We
should evaluate different functions in the MCU separately to understand their impact on current consumption. For example, we must be very careful when integrating undervoltage protection. Undervoltage may occur when a battery is inserted or the power supply drops below the normal power supply specification but above the ground voltage. Most MCUs have integrated undervoltage protection, but this will increase the average current consumption by 20uA to 70uA. When selecting an MCU, be careful to avoid increasing current consumption when adding protection mechanisms.
High-performance analog
When making architectural decisions, we must consider analog requirements. We must carefully study the peripherals provided by the MCU product family to ensure that it can meet various current and future needs. For example, some products in the MSP430 family include 12-bit analog-to-digital converters, 12-bit digital-to-analog converters, and low-power op amps, making them ideal for portable measurement applications. When selecting an MCU family, make sure that it offers a high-performance peripheral mix so that designers can get a future integration strategy. In addition, if there is no MCU that offers the right analog peripheral mix, we can also use external analog peripherals, after all, performance is more important than integration.
If the MCU integrates the peripherals, then we need to consider the peripheral design issues to ensure that it fully plays its inherent role in the application. Some CPUs process data more efficiently. A 16-bit MCU with a 12-bit analog-to-digital converter processes data faster than an 8-bit MCU. A 16-bit MCU can sample in one 16-bit register, while an 8-bit MCU has to process samples in two 8-bit registers.
User Interface
Another factor in the design is the efficient integration of all user interfaces required for the application, including keyboards, displays, and communication ports. The keyboard function is very simple, but the designer must ensure that the application can interrupt and efficiently process keyboard keystrokes. Liquid crystal displays (LCDs) are often used to provide feedback to the user, and they are low cost and low power. Most manufacturers customize displays to meet the designer's needs, and no matter what characters and symbols are displayed, they try to minimize the voltage used by the system and the display. When selecting an MCU, make sure that the CPU does not need to update the display regularly. Designers should understand how many segments the MCU can support and how many segments the application can support.
Communication ports are another user interface. We can use a variety of communication schemes, including I2C, RS-232, RS-485, and RF. We need to carefully consider how to implement communication technology in the MCU. We can choose software and hardware based on the required baud rate, including low-cost software solutions, but they usually take up some resources of the MCU.
The MSP430FG43x series is a good example that reflects all of the issues we have mentioned above (see Figure 1). In this example, a 12-bit analog-to-digital converter samples a high-precision voltage source sensor with a resolution of 4096 steps, and the sensor signal has been pre-amplified before sampling to meet the required dynamic range. The non-inverting input of the preamplifier is biased by a 12-bit analog-to-digital converter for precision trimming. The system is battery powered, and a supply voltage monitor (SVS) circuit resets the application when the supply voltage drops below the specified minimum voltage. An LCD display displays the measurement results to the user.
Table 1.0
Microcontroller Selection Checklist
Extended Battery Life:
Minimized power consumption in various operating modes in real-world applications
Minimum CPU wake-up time
Interrupt-driven architecture
Minimum leakage current
Peripherals can operate without CPU intervention
Functions designed to reduce power consumption (for example, no additional power consumption during brown-out reset)
High-Performance Analog:
The product family supports a different combination of high-performance peripherals
Don't sacrifice peripheral performance for integration
Use peripherals that do not consume CPU resources
Use a CPU with a higher data sampling efficiency, such as a 16-bit architecture
User Interface:
Interrupt-driven port for keyboard
Use low-power LCD display
Select hardware and software communication ports based on baud rate requirements
Figure 1. The MSP430FG43x family integrates high-performance data converters and operational amplifiers optimized for typical portable measurement applications