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Cheap, ultra low power RTC to periodically wake microcontroller from power-down?

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I wanted a rude username:

--- Quote from: Peabody on December 04, 2019, 03:40:47 pm ---The power switching circuit would just be a P-channel mosfet, an NPN and a couple resistors, and use one GPIO pin.

--- End quote ---

Or run the target device directly from the GPIO.  ;D

Wimberleytech:

--- Quote ---
Well, yeah. I2C is definitely more complex to implement and takes more logic. I can't see how it would be "cheaper". Besides, if I'm not mistaken, I2C used to be licensed, so you had to pay for a license to include it in an IC. I don't know for sure whether it's still the case or if it's in the public domain now?

--- End quote ---

No, pretty sure the Philips patents have expired.

iMo:
The lowest power afaik is the PCF8563 (250nA @3V).
A CR2032 150mAH will last for 68 years  :D

SiliconWizard:

--- Quote from: Wimberleytech on December 06, 2019, 12:46:29 am ---
--- Quote ---
Well, yeah. I2C is definitely more complex to implement and takes more logic. I can't see how it would be "cheaper". Besides, if I'm not mistaken, I2C used to be licensed, so you had to pay for a license to include it in an IC. I don't know for sure whether it's still the case or if it's in the public domain now?

--- End quote ---

No, pretty sure the Philips patents have expired.

--- End quote ---

I took a look. https://en.wikipedia.org/wiki/I%C2%B2C

1. No license fees required since Oct. 2006 (probably explains why the number of ICs with an I2C interface has risen significantly in the last 10 years or so);
2. NXP apparently still has a program for allocating slave addresses, and there are fees, although I couldn't find any precise information on this. According to the I2C specs, "Assignment of addresses within a local system is up to the system architect", thus we can understand that registering slave addresses is optional and not required. I guess most vendors don't bother to register these days, all the more that there are only a very limited set of possible addresses, and that the usual addresses used for various types of devices are relatively easy to find.

For anyone interested, the latest rev. specs: https://www.nxp.com/docs/en/user-guide/UM10204.pdf

I wanted a rude username:

--- Quote from: imo on December 06, 2019, 10:41:20 am ---The lowest power afaik is the PCF8563 (250nA @3V).

--- End quote ---

For 20¢ per unit in volume (or 10¢ for the HuaGuan Semiconductor PCF8563M, a copy/clone claiming identical characteristics), this is perfect! It can be used in countdown mode without ever needing to set the time and date, and the countdown interrupt can be set to either latch on or reset. The maximum countdown duration of 4.25 hours is certainly acceptable since the microcontroller can wrap it in a counter loop for longer durations. A brief I2C transmission would only be required when changing the countdown duration.

https://www.nxp.com/docs/en/data-sheet/PCF8563.pdf
https://datasheet.lcsc.com/szlcsc/1910291305_HGSEMI-PCF8563M-TR_C434478.pdf

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