Microcontroller chips are increasingly finding their way into more portable appliances as designers learn how to use them to make less expensive products. Typically, selecting a chip depends on cost and availability, and how easily a designer can get development tools. Other significant factors include details of chip architecture such as number of pins, and input and output features. But perhaps the most critical design goal is how to squeeze the last available milliwatt of power from the appliance’s batteries.
The cost of a power supply is generally influenced by the power consumed by the microcontroller. Power usage increases with clock speed, so balancing the number of functions with the lowest possible clock rate can significantly improve battery uptime between charges or changes.
Many new microcontrollers come with a low-power standby or sleep mode that helps reduce this current drain. The mode switches off the clock oscillator when the appliance is not used and wakes it up again when some external event such as a voltage change or a pushbutton triggers the device.
In spite of these conservation methods, finding a suitable battery is not easy. Users of mobile appliances expect disposable batteries to be cheap, last a long time, and be readily available. For the perfect “out-of-box experience,” buyers also want the batteries to be included. Such demands usually limit the choices to 9-V batteries, miniature 12-V lighter batteries, or four 1.5-V AA cells.
A serious problem is that most microcontrollers run on 5 Vdc, so it requires an additional external regulator to reduce these higher voltages. But some new microcontrollers now come with built-in voltage regulators which can significantly reduce the total cost and complexity of numerous low-power products.
The output ports are also an important consideration in selecting a microcontroller. Microcontrollers cannot directly switch the high power loads typical of appliances. Moreover, some loads require ac voltage while others need dc. Triacs or relays normally switch 110 to 220-Vac loads, and for all dc-powered applications, power Mosfets are most often used for their low on resistance which also reduces power dissipation.
Selecting a microcontroller that can handle up to 12-Vdc outputs eliminates high-voltage external drive circuits for Mosfets thereby reducing energy loss and excessive heat in high-current stages. For example, a high-voltage microcontroller and Mosfet can replace the high-current mechanical on/off switch in a cordless vacuum cleaner. This approach optimizes battery life as well as motor life and provides motor speed control, battery charge/discharge monitoring and display, motor overload monitoring, and dc motor start sequencing.
Input signals also affect power consumption. Signals enter the chip through a buffer structure to trigger an action or a response. When a slowly rising voltage is applied, the upper switch turns partially on before the lower switch fully turns off. The problem is, with both devices conducting, the leakage current can drain the battery and heat the chip. To get around this, a current limiting element inserted between the switches acts as an inexpensive threshold detector for slowly varying voltages. When this input structure is included, a low-current relaxation oscillator can provide the timing, voltage/resistance measurements, and periodic wake-up calls from sleep, as in a remote temperature-sensing unit.
Although not tied directly to the power drain, the instruction set is another critical factor in selecting a microcontroller. In most cases, the simpler, the better. This is why reduced instruction set (RISC) architectures are becoming more widely used. A simple instruction set simplifies writing, debugging, and software maintenance. RISC-based instruction sets have little redundancy and make more efficient use of program memory. Most programmers can learn the entire instruction set and start writing code in only a matter of hours.
Information for this article was provided by Jan van Niekerk, Microchip Technology Inc., 2355 W. Chandler Blvd., Chandler, AZ 85224, (602) 786-7200