New-Tech Europe Magazine | July 2017

205 days, before triggering an interrupt to wake up the MCU (See Figure 1). Of course, it is rare for an application to have a need to remain idle for such long period, but the capability is present if necessary. When using an internal low power (31kHz) oscillator to feed the resulting (long) timer, the lowest cost solution can be achieved. But the power consumption of this implementation can be reduced further by 50% to around 2.3 µA, by using an external 32kHz crystal connected to the secondary oscillator (SOSC) at a slightly higher cost.. External interrupt sources can also be used to wake up the microcontroller, such as a switch or sensor. Some of the larger MCU/MPUs have multiple interrupts with priority levels, but these features are often not present in most of the low pin count MCUs on the market. Remember the Configurable Logic Cell module that we used to extend the timer period in the previous example? Not only can it be used to create extra interrupt sources when the MCU has only one system INT, the CLC also allows designers to add conditional or sequential logic to the wake-up routine; making it smarter with no additional current draw. If the system requires a number of signals to represent a specific state, in order to wake up the CPU to check the condition, very often it was awakened due to one signal change, only to find the other signals had not yet occurred. It is now possible to configure and combine the available logic functions and state machines in the CLC, or even multiple CLC modules, to create specific wake- up conditions that avoid frequent false triggers and unnecessary power drain. The device wake up time can be a large component of the “running” time in the periodic profile of an application. While for an 8-bit microcontroller internal oscillator this is typically limited to a handful of clock cycles (3-5 cycles

Figure 1: An 8-bit microcontroller’s core-independent peripherals are being used to extend the idle period to 205 days, without any additional components or power penalty

roughly every four hours to read the humidity sensor before going back to sleep again. Does this mean that we must wake up more frequently, due to the internal timer limitations? Not necessarily. One option is to use a Real Time Clock (RTC) and crystal that can provide accurate time keeping in terms of hours, days, months and even years, if needed. Since not all microcontrollers offer an integrated RTC and crystal, often for cost reasons, a stand-alone RTC can also be considered. Core-Independent Peripherals Another option is to extend the idle period without any additional components or power penalty is to use the unique peripherals found in some next-generation 8-bit microcontrollers (such as Microchip Technology’s PIC® MCUs). For example, designers can connect one of these MCU’s Configurable Logic Cells (CLCs) and its Numerically Controlled Oscillator (NCO) to the 16-bit timer, in order to extend the period from 17 seconds to

performance. This is where an 8-bit microcontroller can outshine a 32-bit device in many cases. Some 8-bit MCUs consume as little as 20 nA in the lowest power setting, while 32- bit bottoms out around 10 to 20 times higher, at best. There are a number of ways to wake up the microcontroller from sleep mode. Using the microcontroller’s internal timers to wake up the system periodically is a common practice. The timer can be configured to trigger an interrupt after it overflows. A 16- bit timer with 1:8 pre-scaler, running off the internal, low-power 31 kHz oscillator (or with an external crystal) can keep the device in sleep for about 17 seconds. Another option is to use the MCU’s Watchdog Timer (WDT), ideally with a maximum idle time of 256 seconds while consuming around 440 nA. (Once again, a typical 32-bit MCU with WDT enabled consumes at least 3 times the current of an 8-bit MCU.) Take an application that doesn’t need to wake up frequently, such as an environmental monitor that wakes up

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