| Electronics > Projects, Designs, and Technical Stuff |
| Cheap, ultra low power RTC to periodically wake microcontroller from power-down? |
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| GeorgeOfTheJungle:
--- Quote from: imo on December 06, 2019, 10:41:20 am ---The lowest power afaik is the PCF8563 (250nA @3V) --- End quote --- --- 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. And then you wouldn't really have to worry about sleep current or a regulator's quiescent current or current used in dividers since the whole thing would only be powered up briefly. --- End quote --- :clap: I think I'm going to borrow the idea :) the PCF8563 /int pin can drive the high side switch :-+ |
| I wanted a rude username:
And as its final action, the microcontroller clears the timer flag. How would you switch on the MOSFET after a power-on/reset though? |
| floobydust:
I had a datalogger product using PCF8583T and it was unreliable for the wakeups. Set the alarm and no interrupt occured, some of the time. The flag just didn't get set. Found the silicon errata sheet and screamed for quite a while at that bug in 118 or 112. So I will never again use a PCF family RTC. They are cheap because they are primitive, quirky state-machines that need careful software considerations. Enjoy how complicated the read events are: NXP Semiconductors AN10652 Improved timekeeping accuracy using external temperature sensor PCF8563 More info: NXP RTC UM10301 User Manual for NXP Real Time Clocks PCF85x3, PCF85x63, PCA8565, PCF2123, and PCA21125 |
| I wanted a rude username:
--- Quote from: floobydust on December 06, 2019, 11:03:15 pm ---I had a datalogger product using PCF8583T and it was unreliable for the wakeups. Set the alarm and no interrupt occured, some of the time. The flag just didn't get set. --- End quote --- I can't find the errata (do NXP publish them?), but did the bug occur because the alarm was used in seconds mode, and the microcontroller's reads/writes of the time/date registers were causing the PCF8583 to stop the clock for one second (to allow atomicity) and thus miss the interrupt? |
| iMo:
The key message with this kind of RTC is: "when the read or write I2C operation takes more than 1 second you loose 1 second". Normally your read or write I2C operation takes a few milliseconds with 100kHz I2C clock.. PS: 1 second is lost because the I2C operation stops the internal "seconds" counter thus if the operation is longer than 1 second your RTC will show 1 second less. |
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