ARM MCU built in sub-threshold technology for 10-fold power reduction

ARM MCU built in sub-threshold technology for 10-fold power reduction

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By eeNews Europe

Ambiq Micro, based in Austin, Texas, says it has resolved the issues surrounding placing logic built with sub-threshold technology into volume production in a standard CMOS process. Its first announced products are the Apollo family of four 32-bit ARM Cortex-M4F microcontrollers (MCUs). In real-world applications, their energy consumption is typically 5 to 10 times lower than that of MCUs of comparable performance, resulting in far longer battery life in wearable electronics and other battery-powered applications. The reduction in energy consumption is achieved using Ambiq’s patented Subthreshold Power Optimized Technology (SPOT) platform.

In sub-threshold logic, individual transistors are never turned completely on; effectively, logic levels are propagated as leakage-current or leakage-current-plus-a-bit. Dissipation is greatly reduced but the generation and detection of logic levels, and execution of logic functions, has to be stable in the face of process variations, noise, temperature drift and a range of other variables. Previous successful attempts to use sub-threshold logic (watch ICs, for example) have most often been built in in-house, dedicated fabs where process variables have been constantly tuned and refined. Ambiq’s claim is that it has designed structures that can maintain correct operation when implemented in a stock foundry CMOS process.

“We do have to do work at the RTL level,” says a company spokesman, “But the IP of the ARM MCU is untouched – this is a standard ARM core, with the floating point unit.” All of the benefits to power flow from the sub-threshold implementation.

Wearable devices that might otherwise run for days or weeks on a battery can be designed, or redesigned, Ambiq says, to function for months or years. Engineers can take advantage of the expanded power budget to add new features and functions that were not previously possible, or opt for smaller batteries.

Apollo MCUs optimise both active and sleep mode power (the two factors are mutually contradictory in a conventinal process, the company explains). They consume 30 µA/MHz when executing instructions from flash and feature average sleep mode currents as low as 100 nA. This extremely low energy consumption does not compromise performance. The ARM Cortex-M4F core, with its precision floating point unit, offers the computational horsepower required in an Internet of Things (IoT) world where algorithmic processing requirements are increasing due to the growing use of sensors, audio, and automation sources.

Apollo MCUs operate at up to 24 MHz. They are available with up to 512 kB of flash and 64 kB of RAM to accommodate radio and sensor overhead in addition to application code. Communication with sensors, radios, other peripherals and an optional host processor is implemented via I²C/SPI ports and a UART. On-chip resources include a 10 bit, 13-channel, 1 Msample/sec ADC and a temperature sensor with ±2ºC accuracy. Two compact packaging options are available – a 64-pin, 4.5 x 4.5 mm BGA package with 50 GPIO and a further size-optimised 2.4 x 2.77mm, 42-pin CSP with 27 GPIO.

Ambiq Micro’s SPOT platform operates transistors at subthreshold voltages (less than 0.5V), rather than using transistors that are turned all the way “on” at 1.8V. It uses the leakage current of “off” transistors to compute in both digital and analogue domains. The company’s AM08x5 and AM18x5 families of ultra low power real time clocks, launched in 2013, are based on the same platform.

Asked if the 24 MHz clock speed was a limit, Ambiq’s spokesman says that there is scope to increase it further but that the process’ limits are in the “10s of MHz” – “this will never be a Ghz-processor, server-class platform – its natural area of application lies in the lowest power, the portable and battery-powered [space]”.

Ambiq Micro’s CEO and President, Mark Foley, said: “The energy efficiency of microcontrollers has been getting better over the last few years but nobody has delivered the near order-of-magnitude improvement made possible by our SPOT platform. The technology, proven in our real-time clocks over the last couple of years and now applied to Apollo MCUs, delivers the breakthrough in battery life that designers of portable devices have been demanding. Looking ahead, we predict that semiconductor energy consumption will be halved every two years.”

Apollo MCUs are sampling to selected customers now. Volume production will commence in the spring of 2015 with prices from $1.50 (10,000).

Ambiq Micro;

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