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We generally take it for granted that an MCU is a digital device. However, the newest generation of ultra-low-power MCUs with their on-chip low-power peripherals are excellent examples of a product which truly mixes analog and digital circuits in the same device. These modern MCUs should probably be called microsystems because they indeed implement systems on a chip.

The first thing that comes to mind with an MCU is the CPU. While you may think this was -- for certain -- a digital circuit in fact, to achieve the lowest power operation, analog techniques must be used in its development. Because of this the design may not be synthesized, in which case it would be treated more like an analog cell than a digital core. Another benefit of this type of design is the larger operating voltage range. In reality the CPU is only a small part of a typical MCU device.

In battery-powered applications long battery life is critical. There are two factors which are important in determining the battery life: The duty cycle (the amount of time the unit will be in the active mode vs. the time in the standby mode) and the time it takes to go from low power to active operation. The standby power consumption is typically more than one hundred times lower than the active power consumption. A long activation time can force the system to stay in the high-power mode for hundreds of milliseconds before activation.

A typical MCU can take more than 15 ms to go from low power to fully operational. With the right design technique this can be lowered to a few microseconds. In many applications this can lead to significant power savings especially where the MCU must wake up, perform a few operations such as data logging, and then go back to sleep. If this operation must run several hundred times per second, a typical microprocessor would need to be kept in active mode 100% of the time to accomplish the same task.

Proper analog clock circuit design is key to a quick startup time. The cheapest way to generate an accurate clock is to use a 32 kHz crystal and, while the MCU may not require external components, the designer must still be careful about the board layout to ensure proper loading of the crystal. In older microprocessor designs two crystals were needed for high-speed and low-speed operation but with the newest MCUs the high-speed clock can be locked to the 32-kHz crystal without additional expense. Good phase-locking design techniques can enable high-speed startup and a quick wake-up from sleep mode.

By having the CPU control and manage the power to all on-chip peripherals many different low-power applications are possible. For example, a low-power real-time clock can be implemented by coupling the oscillator directly to the timer/counter while turning the power off to all other parts of the system. If the interrupt structure is designed right the timer overflow output can be used to bring the CPU out of its low-power mode when a zero count is reached. Likewise there are benefits of controlling the power to the ADC separate from the rest of the peripherals. If the CPU and other I/Os are turned off during an conversion there is less power-supply noise and therefore a much more accurate result.

Many of the other peripherals in the MCU have analog capabilities. Comparators can be included on what would otherwise be a digital input, changing that input from a digital I/O to an analog function. Coupling this type of simple circuit with the counter/timer, you can implement such things as a slow-speed dual-slope ADC with just a few external components. One application would be to couple this with a thermistor to make a very inexpensive temperature sensor. For systems needing higher-speed conversion, many MCUs have families with an on-chip ADC.

While there are many variations of peripherals on today's MCUs, they do not always have the right combination of on-chip peripherals and external circuits must be designed. To manage the power on these devices external power switch or LDOs may be needed, and with the newest parts not only do you get very accurate regulation with low drop-out you also get the ability to turn the LDO off, while only drawing a minimal 1 µA of power. Even better, the LDOs have a Power Good (PG) output that allows the MCU to know when the power is dropping to a level too low for proper operation.

Texas Instruments has the MSP430 MCU with its on-chip peripherals designed specifically for low-power applications. Its standby current is 1.5 µA with an active quiescent current of 450 µA.

An excellent example of an application requiring this ultra-low power is a utility meter where the controller must have an accurate clock to interrupt the processor at given intervals. The processor must then wake-up and start collecting data (probabbly with an analog-to-digital conversion). With a quick wake-up time it can then go back to sleep and wait for the conversion to complete. It would then wake up again, run an algorithm to determine cumulative consumption, re-enable the timer and go back to sleep.

Today's MCU should not necessarily be viewed as a digital device. Many analog design techniques and capabilities are designed into these controllers and they integrate many functions that have typically been implemented with external analog components. This is only a start; in the future capabilities of these mixed-signal devices will expand as we continue our progress towards more complex systems-on-a-chip.

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