Applying subthreshold voltage technology to real-time clocks changes the MCU power equation

Figure 1: Using a subthreshold voltage CMOS process, Ambiq Real-Time Clocks lower power of all devices in an embedded design.

Figure 2: Due to the use of its subthreshold voltage process, Ambiq’s family of Real-Time Clock circuits, outstrip the capabilities of most competitors.

Figure 3 :Because Ambiq’s RTCs are fabricated using a 180 nanometer CMOS process, they incorporate a much broader range of power management features.

Figure 4: In a typical Vss power switched implementation, where an Ambiq RTC is connected to the MCU or other system logic via an SPI or I2C I/0 channel, the VSS pin of the MCU is switched using the PSW pin. The nIRQ pin is connected to an interrupt input of the MCU, so it can use the RTC interrupt functions when it is awake.

A basic hardware building block in any embedded developer’s repertoire of low power design aids is the lowly real-time clock (RTC). In its simplest form, the function of an RTC is to keep track of the current time. But the RTC has taken on a pivotal role as the means by which to monitor and manage sparse power resources in almost any untethered embedded or Internet of Things design.So it should come as no surprise that almost every maker of microprocessors and microcontrollers also provides at least one RTC circuit in its catalog of products, including most of the major makers of ARM-based processors and SoCs, including NXP Semiconductor, Silicon Labs, and ST Microelectronics. The reason is simple: developers still have to deal with the power requirements of the rest of the components in a design.

Ambiq Micro has addressed this challenge by using the Subthreshold Power Optimized Technology (SPOT) used in its Apollo family of ARM Cortex-M4F MCUs to also fabricate its AM08XX/18XX real time clock family (Figure 1), now in volume production.

In an interview with EE Times, Ambiq Vice President of Marketing Mike Salas said that as a result of the incorporation of SPOT, developers using Ambiq RTCs can reduce the power consumption of any component in a design 10-fold, including the MCU/MPU. He said the company’s tests indicate that using one of its subthreshold voltage-based RTCs in a system will reduce current — and thus power consumption — to no more than 55 nano-amperes with an external crystal, 15 nA with a standard resistor–capacitor circuit (RC circuit), or 21 nA using an auto-calibrated RC.

“The RTCs we have built allow the design of systems that are roughly seven times lower in power than the NXP PCS 2123 RTC, and at least 10 times lower than similar devices from Integrated Device Technology, Maxim, and STMicroelectronics,” he said.(Figure 2).

And because they are processor agnostic, Ambiq’s RTCs can reduce the power consumption of any embedded system design based on any processor, including high end ARM, MIPS, and Intel Atom-based 32- or 64-bit CPUs as well as any 8- or 16-bit microcontroller.

If these claims are proven in working designs, the near-term impact of Ambiq’s RTC family is more far-reaching than the use of its subthreshold voltage technology in 32-bit processor designs.

The shift to threshold voltage-based RTCs will make any 32-bit MCU more viable in power constrained IoT designs. But at the same time, it will make already low power 8- and 16-bit MCUs even more so, considerably extending their operational life, especially in applications that require unattended operation over months or years using only batteries or energy harvesting. The AM08XX/18XX devices achieve this power savings in a number of ways, Salas said.

First, operating purely as real time clocks, they drastically reduce the independent battery-backup requirements that any RTC has to depend on to provide timing and power management functions to the rest of the system.

“Despite the fact that they play a key role in managing MCU design power resources, traditional RTCs are incredibly power-hungry,” he said. “Because our RTCs draw seven to 10 times less current, they can operate using less expensive and commodity type coin cells and cheaper ceramic capacitors, which only cost a few cents, instead of more expensive super-capacitors.”

Second, to extend the subthreshold-based power savings to the rest of the system, the AM08XX/18XX family of devices can be configured in a variety of system implementations to control the power used by other elements in the system (Figure 3).

The most common use of an Ambiq RTC is to manage the VCC power switch that completely turns off the MCU and/or other system elements. But to provide the MCU system designer additional flexibility, Ambiq’s RTCs can also be reset- or interrupt-driven.

The key benefit of using an AM08XX/18XX device in the recommended VSS power switched mode (Figure 4) is that in addition to putting the MCU – or any other components similarly configured – asleep, it can be used to wake up the MCU or other logic with any number of triggers: a simple predetermined alarm, via a countdown timer to wake it up periodically, or by an externally operated manual input, such as a push button or switch. It can also be triggered by a range of external device and analog inputs, such as light sensors or radio antenna detectors, when the battery is low, or when any interrupt failure occurs.

Because the AM08XX/18XX family is based on an RTC circuit originally developed as a means by which to test Ambiq’s proprietary nanometer CMOS SPOT enhancements, they are fabricated with an 0.18 millimeter (180 nm) process, leaving considerable IC silicon real estate for additional features, versus the much more conservative geometries used in most competitive RTCs.

“This means we were able to integrate a wide range of power management functions not normally included in a real time clock circuits,” said Salas, “including power switch management, system sleep, automatic battery switchover, analog voltage comparator monitoring, reset and trickle charge management.” These additional on-chip features make them viable in a range of use cases where a traditional RTC alone cannot be considered without additional external logic.

For example, using an external power switch feature built into the design, an Ambiq RTC can be used to switch an MCU-based design to an alternative backup power resource. Alternatively, it could be used in combination with other power management functions to power gate on and off a variety of external system functions, including other MCUs, displays, or radio systems.

“In energy harvesting designs, which often use a rechargeable battery or a super-capacitor to store energy for later use, our RTCs’ trickle charge feature would allow collection of energy at much lower voltage and current levels, as well as manage the storage of such resources, and when to use them.”

Bottom line, said Salas, the fundamental advantage provided by the company’s AM08XX/18XX RTCs is that they allow the system designer to ignore the sleep current of other system components, allowing the use of components which under ordinary circumstances would have to be optimized using other less efficient parameters.

— Bernard Cole, MCU and PCB Designline editor,EE Times  is an embedded microsystems technology analyst who writes about hardware/software design and use across the range of consumer, industrial, automotive, networking and Internet of Things applications. He can be contacted via LinkedIn, by email at bccole@techrite-associates.com, or at 928-525-9087.
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Posted in Ambiq Micro