Sub-threshold operation is a way to reduce CMOS power consumption – at the expense of far greater process and temperature-dependent behaviour.

“When compared to a typical circuit operating at 1.8V, a ‘near-threshold’ circuit operating at 0.5V can achieve up to a 13x improvement in dynamic power. An even more aggressive ‘sub-threshold’ circuit operating at 0.3V can achieve up to a 36x improvement,” said the firm. “Operation at such low voltages is problematic due to susceptibility to noise, temperature, and process drift. Ambiq Micro has addressed all of these challenges by redesigning every analogue and digital circuit.”

For even lower power consumption, the real-time clocks have an internal 128Hz RC oscillator that typically cuts drain to 14nA at 3V, or 22nA with a self-calibration circuit keeping “frequency accuracy similar to the XT [crystal] oscillator”, said the firm in the RTC chips’ data sheet. Maximum current consumption its various modes varies between 170 and 330nA.

It is possible to digitally tune the RC and XT oscillators within 2ppm at any particular temperature, although this count will drift slightly with temperature – around 0.1Hz/°C in RC mode.

Power is drawn automatically from either from a Vcc input, or a back-up battery input.

There are four versions:

AM0805 has an I2C bus

AM0815 has SPI

AM1805 has I2C + extra functions

AM1815 has SPI + extra functions

It is not clear if all of the above are yet in production.

The latter two – AM18x5 – also include three other hardware blocks to reduce the power consumption of an associated microcontroller: reset management, an integrated 1Ω open-drain power switch, and a system sleep manager.

By using the power switch and sleep manager together, the negative rail to the associated MCU can be completely cut.

Ambiq is planning sub-threshold ARM Cortex-based microcontrollers, to be branded Apollo.