Electronics > Projects, Designs, and Technical Stuff
Cheap, ultra low power RTC to periodically wake microcontroller from power-down?
Peabody:
--- Quote from: I wanted a rude username on December 06, 2019, 10:56:48 pm ---And as its final action, the microcontroller clears the timer flag.
How would you switch on the MOSFET after a power-on/reset though?
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Or the microcontroller could set its own next wakeup time before shutting down its power.
The mosfet would be turned on by the RTC module, which would power up the microcontoller. Attached is my circuit for a mailbox notifier which has the power completely shut down until the door is opened, which turns on the mosfet. The microcontroller powers up, sends the notification message, then shuts down its power. The RTC module's open-drain /INT pin would take the place of the magnetic switch. The module would be powered seperately by a coin cell, with common ground with the main battery. Open-drain is perfect for this.
I wanted a rude username:
Thanks, that 100 kOhm pull-up explains how it starts without the interrupt line being asserted. But doesn't that cause a constant 30-45 µA current draw through the pull-up?
GeorgeOfTheJungle:
--- Quote from: I wanted a rude username on December 06, 2019, 10:56:48 pm ---How would you switch on the MOSFET after a power-on/reset though?
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You just have to momentarily GND the mosfet gate with a pushbutton to turn it on manually. And an open collector gpio to keep it ON the rest of the time.
--- Quote from: I wanted a rude username on December 07, 2019, 12:48:11 am ---But doesn't that cause a constant 30-45 µA current draw through the pull-up?
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Through Q2 to GND you mean?
I wanted a rude username:
Through the normally-closed reed switch. Seems like the leakage would outweigh the power saving of this design.
--- Quote from: GeorgeOfTheJungle on December 07, 2019, 12:51:08 am ---You just have to momentarily GND the mosfet gate with a pushbutton to turn it on manually. And an open collector gpio to keep it ON the rest of the time.
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Could it be done without a switch, so that the microcontroller starts as soon as the device is powered?
GeorgeOfTheJungle:
--- Quote from: I wanted a rude username on December 07, 2019, 01:05:09 am ---Could it be done without a switch, so that the microcontroller starts as soon as the device is powered?
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Yes, with a capacitor in parallel with Q2 C-E.
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