Requirements for increasing computing power and more integrated functions are driving a growing number of applications from 16-bit to 32-bit microcontrollers.

This is equally true for battery powered applications, which benefit from the lower voltage supply, as well as the high performance and small die size achieved by 32-bit devices that are based on advanced CMOS process.

However, deep submicron technologies also have a very important drawback: their much higher leakage is a major issue, especially for the limited power resources of a battery powered application.

To enable migration, new 32-bit microcontrollers, including general purpose devices, must provide very efficient ultra low power modes for long term standby.

In this article I will describe how STMicroelectronics engineers enhanced its Cortex-M3 core-based STM32 microcontrollers with low power modes and features that mitigate the impacts of leakage on battery powered applications where static current may be a major contributor to consumption.

Specifically I will address  innovations in "ultra low power standby" and Real Time Clock implementation in our STM32 ARM Cortex-M3 core-based microcontrollers.

Leakage current
The leakage can be defined as the remaining continuous current in a CMOS gate in static state (no switching activity). It has several root causes, and increases with each new technology shrink. Its three main contributors are sub-threshold, gate, and junction tunneling leakage.

Figure 1: Leakage currents in a CMOS transistor

Sub-threshold leakage is linked to the threshold voltage diminution that is required with the decreasing voltages used in each new technology. Gate leakage is induced by the scaling of the gate oxide thickness that is needed to reduce the "short channel" effect. Junction tunneling leakage is induced by the electric field across a reverse biased p-n junction (tunneling of electrons).

Leakage increases as temperature rises mainly due to the exponential increase of the sub-threshold leakage over temperature. Without any switching activity, the quiescent current of a 32-bit microcontroller using advanced process can still be limited to a few microAmperes at ambient temperature.

However this leakage will rise with temperature and can even exceed one milliampere at 125°C. For this reason, it is very important to take into account the leakage at the maximum application temperature.

Figure 2: Leakage increase with temperature

Even though several techniques exist to limit the leakage of a digital library (increase poly length above minimum allowed by the technology, increase gate oxide thickness on transistors), such modifications impact the propagation time in the digital cells. Using such a library in the entire core logic would prevent the device from achieving high performance in run mode.

The added dilemma for today's 32-bit devices is that, from a structural point of view, the main contributors to leakage current in a microcontroller are digital logic and memories.

In addition to the increasing leakage due to technology shrink, both the digital gate count and average memory size have increased dramatically in subsequent generations of 8-bit, 16-bit, and new 32-bit microcontrollers.

As a result, leakage is a major problem for all general purpose microcontrollers using the latest semiconductor technology and is a particular consideration for battery powered applications with their limited power resources.

Impact of leakage on battery lifetime
Static current consumption becomes the main contributor to average current as soon as the average run time becomes very low compared to the standby time. Given the energy level provided by a battery, a quick estimation of the application lifetime (not including non-linearity of the battery capacitance described by the Peukert law) is:

For example, if the specific ultra low power standby mode was not available on the STM32 128K flash microcontroller the average current could be significantly impacted: typical run current at 72MHz with all peripherals enabled is only 36mA (0.5mA/MHz) thanks to the ARM Cortex-M3 architecture and low power design techniques.

However, due to the use of advanced process, the leakage current starts to be significant at 55°C. Thanks to a very low power voltage supervisor and regulator, static current is still limited to 50 microAmperes at 55°C.

This is negligible compared to the run consumption. However if the application runs only one minute a day, the static current becomes the main contributor to consumption (64%).

To address this problem, the designers of the STM32 went to great lengths to enable a true low power standby by implementing an embedded regulator, independent voltage domains, and integrated power switches at the architecture level. This implementation enables low power modes that can optimize battery life depending on the application.