Zero Power or Low Power Standby? - Page 1
Forum section: electronics / projects

author: NivagSwerdna
author_rank: Super Contributor
timestamp: March 21, 2017, 10:34:48 am
content: I would like to create a very simple circuit that operates a microcontroller for a period of time once a button (actually one of two buttons) is pressed. The microcontroller will decide when to go back to off mode.Is it possible to use one of the button presses to provide sufficient bootstrap power to get the microcontroller started (and to then turn itself on)?The button will rarely be pressed... probably once every month and then the circuit will operate for <24hrs before turning off.or... is this a complete waste of time and I should just deep sleep the microcontroller and use the buttons to wake it up.(Turning peripherals off during sleep)Opinions?[I haven't selected a device yet but I have 6V supply from 4xAAs]

author: Ian.M
author_rank: Super Contributor
timestamp: March 21, 2017, 11:01:14 am
content: It depends. If you can get the average sleep current down below 100uA, it will be good for over three years asleep off fresh alkaline AAs at a nominal 2800mAH capacity - that's about half the shelf life so there's not much point going going lower for a once a month usage pattern.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 21, 2017, 11:08:02 am
content: Microcontrollers often provide deepest sleep modes with only a few uA of leakage, which is totally irrelevant as the battery leakage tends to dominate. It can still wake up from a simple button press interrupt, which makes things very easy. Just keep in mind how all the other circuitry behaves, including capacitor leakages.

author: bktemp
author_rank: Super Contributor
timestamp: March 21, 2017, 11:18:07 am
content: Using the deepest sleep mode is probably the easiest solution: The current consumption is often insignificant.A simple, but not the best solution is connecting the button the the reset pin: When the button gets pressed, the microcontroller resets ans starts the program. When it is done, it goes into the sleep mode.Many microcontroller also support wake up from an external interrupt pin. That is a better solution than using the reset pin, because it ignores additional button presses.If you want a true power down, you could use a transistor in the power rail, turned on when the button is pressed. In addition to the button the microcontroller also controls the transistor and keeps it on until it is done.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: March 21, 2017, 11:36:56 am
content: Quote from: bktemp on March 21, 2017, 11:18:07 amThat's the sort of thing I was musing on... the button charges a capacitor to keep the transistor on long enough for the microcontroller to wake up and then power the transistor... I imagine the need for some diodes since either button can turn the device on but subsequently the microcontroller needs to be able to distinguish which button was pressed (either at wake-up or subsequently during the awake period).

author: Ian.M
author_rank: Super Contributor
timestamp: March 21, 2017, 11:51:16 am
content: Active high buttons, with pulldown resistors. All wakeup capable buttons have a diode to the MCU's Vdd pin. MCU turns on Vdd via a high side PMOSFET driven by a NMOSFET controlled by an I/O pin as soon as it starts, taking over from the supply via the diode of the pressed button and then reads the buttons normally. Its a good solution if you cant get the MCU's quiescent current low enough, or you have a much more limited battery capacity.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 21, 2017, 12:00:06 pm
content: The point is, the power switch transistor is already there - it's integrated in the microcontroller. It is controlled by using the sleep modes.You can replicate everything by adding external circuitry, and either land at the same 1-2uA consumption, or slightly better, or slightly worse.Depends on MCU, though.Controlling the supply with a PFET usually only makes sense if the MCU interfaces with other circuitry not providing sleep modes - although, most modern ICs do provide them!

author: Ian.M
author_rank: Super Contributor
timestamp: March 21, 2017, 12:09:07 pm
content: Yes. If the MCU's sleep mode is low enough power relative to the battery capacity and self-discharge, it doesn't make any sense to add an external self power control as I described. Better to use the PFET directly controlled by the MCU to control the power to any other circuits that don't have ultra low current sleep modes.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: March 21, 2017, 12:34:49 pm
content: Quote from: Siwastaja on March 21, 2017, 12:00:06 pmI'm catching on. Thanks.