Folks,
Here's a follow up to my earlier topic on Ultra Low Power Microcontroller Design:
https://www.eevblog.com/forum/projects/happy-nerd-year!-my-first-project-of-the-year-semi-fail/
That old version managed about 5.4uW (1.8uA at 3.0V).
TL;DR:
This new version managed an average 693nW for a 4-digit clock, and 896nW for a 6-digit clock.That's
385nA at 1.8V and
498nA at 1.8V respectively.
(Note might still be rendering higher resolutions for a few hours after this posting)
The MCUs used are PIC16LF1907 (4-digit) and PIC16LF1947 (6-digit). The LCD is DE126-RS-20/7.5.
The key tricks over and above the first iteration were:
o Use an external RTC with push-pull programmable clock out (MicroCrystal RV-3028-C7)
o Decrease microcontroller voltage to 1.8V
o Find an LCD that works at 1.8V (not hard, I discovered it by accident, I don't think they're uncommon)
o Use an MCU with sleep current of 10s of nA rather than 100s, while allowing key peripherals like LCD to run
The MCU is run at 16MHz when out of sleep, the lowest uA/MHz achievable with the internal clock and fastest wake-from-sleep (~10us).
The RTC's CLKOUT operates at 2048Hz fed into the PIC's T1CLKI, used for both the LCD peripheral and Timer1. Using the external RTC reduced the raw 32kHz requirement from 170nA to 45nA: using the RTC's 2048Hz output rather than the PIC's 32kHz SOSC for timing and LCD reduced the dynamic current in the PIC by a further couple of hundred nA.
Active duty cycles are 2,000,000:1 for the 4-digit and 16,667:1 for the 6-digit.
To allow use from a 3.0V button cell I recommend either the MCP1810T-18 (now EOL) or the TPS7A02-18 LDOs. These are 20nA and 25nA Iq respectively. Unfortunately these 20nA LDOs, when used with 3.0V button cells rather blows the 1uW, but I'll keep upbeat about it, the clock itself is well under 1uW. We just need a battery chemistry with a nominal ~2.0V or so. A couple of lemons maybe?