About low power solutions, MCU and RTC - Page 1
Forum section: electronics / projects

author: sainter
author_rank: Regular Contributor
timestamp: September 29, 2016, 11:22:28 am
content: Hi,For a long time, I'm having a nagging thoughts about low power, long lasting ,battery powered devices. There are a lot of sub uA sleeping current MCU out there, but as far as I understand they all have wake up trigger on High logic. So my question, what would be the best way to periodically (1/s, 1min, 1/h) signal an MCU in deep sleep, to do some work.The device that caught my eye, was ST M41T6x RTC, witch has 350nA timekeeping current with alarm option. Did anyone use RTC for MCU wake up and how far from spec. did the RTC current go?Thanks for your thoughts.

author: KL27x
author_rank: Super Contributor
timestamp: September 29, 2016, 12:23:40 pm
content: Unless you really need RTC, you can just use the micro's internal WDT to wake it up periodically. With prescaler, you can have that happen in minutes or whatnot. First thing on waking increment a post scaler register(s).... until it hits X, immediately go back to sleep... and you can stretch that to years... +- 1 or 2%, of course, without a RTC. Duty cycle of 0.0001% is going to save you more current than a RTC running the entire time and waking it up just the once a year.

author: Wilksey
author_rank: Super Contributor
timestamp: September 29, 2016, 12:35:27 pm
content: http://ambiqmicro.com/

author: sainter
author_rank: Regular Contributor
timestamp: September 30, 2016, 05:59:44 am
content: But if the WDT is left on in deep sleep, MCU has to have the oscillator left on. So the current draw of the MCU would be more then without WDT and osc. + the RTC. As far as I understand, in low power application nA does meter? Or is it lost in self discharge of the battery and the peaks of on time current?

author: viperidae
author_rank: Frequent Contributor
timestamp: September 30, 2016, 06:41:50 am
content: An atmega328 runs at 5uA with the wdt running.An msp430 can get down to 0.7uA while running its internal rtc.St has arm mcu's with 0.54uA sleep + rtcI'm sure there are other lower power mcu's out there too.

author: ebclr
author_rank: Super Contributor
timestamp: September 30, 2016, 06:44:10 am
content: Energy-Friendly Microcontrollers (MCUs)http://www.silabs.com/products/mcu/Pages/default.aspx

author: JPortici
author_rank: Super Contributor
timestamp: September 30, 2016, 07:43:03 am
content: Quote from: sainter on September 30, 2016, 05:59:44 amthe WDT might have its own low power oscillatorfor example, in most if not all pic with internal oscillator there are two of them, the HFINTOSC and the LPINTOSC.PIC16LF1823 (to keep things easy, it's a bit more complicate with other chips)HFINTOSC: 8 MHz nominalLPINTOSC: 32 kHz nominal30 nA base current @3V, room temperature500 nA LPWDT current @3V, room temperaturethe accuracy of the LPOSC is not so great (i have two identical prototype of an RF remote built around this pic, WDT period is 32s nominal and they drift apart 1s every 10 days or so)and the current will be HIGHLY dependant on the temperature, much much more than Vddbut if you don't need too much acuracy it's not hard to get sub uA current consumption during sleep. the datasheet can be your friend to get you to at least a figure of merit, then you will always have to build and test your prototype(s)those new sillabs chips are rather interesting over the consumption as well, i am considering redesigning the said rf remote from a pic+transceiver to a EZR32HG. promised sleep consumption and sensitivity are much better at a maybe lower cost. i guess i'll see.