Electronics > Projects, Designs, and Technical Stuff
Low Voltage Detection & Power Cut IC
mark03:
Depending on your ARM MCU, you might be able to do brownout detection there with no additional parts needed. I know that some of the STM32 families, for example, have a lot of configurability where you can get interrupts at different voltage levels.
Gribo:
A MAX809 might do the trick. It is ~0.8$ at Digikey. You have several threshold voltages to choose from. Also, make sure the CPU clock has the correct setting for the supply voltage. Usually, you can't run at the highest clock rate when the voltage is low.
JDW:
--- Quote from: mikerj on August 27, 2019, 03:57:54 pm ---If a microcontroller has no internal brownout/reset controller or a buggy/inadequate one then it's quite possible for a a micro to start doing unpredictable things during a slow supply voltage rise/fall, including executing from random bits of program memory that may contain the flash erase/write functions.
--- End quote ---
I don't think the Brown-out feature of my PIC16F1508 (which I am using) is "inadequate" but it applies to my PIC and not the Fingerprint Sensor. it is the Fingerprint Sensor that, according to the manufacturer, needs to be powered off (since there's no way to easily "RESET" it) when the sensor's input voltage falls below 2.7V.
--- Quote from: Gribo on August 27, 2019, 07:44:48 pm ---A MAX809 might do the trick. It is ~0.8$...
--- End quote ---
Page 2 of the MAX809 datasheet in the Absolute Maximum Ratings section does mention that the Output Current of the Reset pin is up to 20 mA. But are these reset chips really best for my application where I would need to use the Reset output to power the Base of a BJT PNP transistor in order to cut +3.3V power to my fingerprint sensor (to completely power-off that sensor)?
Because the fingerprint module is a self-contained unit from the factory, I cannot expect an assembly factory to hand-solder a tiny wire to multiple reset pins on the various SMD IC’s of that fingerprint sensor, and that is why I would need to cut power to the entire sensor if the voltage falls below a predetermined threshold.
mark03:
--- Quote from: JDW on August 28, 2019, 12:00:30 am ---I don't think the Brown-out feature of my PIC16F1508 (which I am using) is "inadequate" but it applies to my PIC and not the Fingerprint Sensor. it is the Fingerprint Sensor that, according to the manufacturer, needs to be powered off (since there's no way to easily "RESET" it) when the sensor's input voltage falls below 2.7V.
--- End quote ---
Ah, I thought you said you were using an ARM. In any case, there are various MCU-based approaches that might not require a separate chip. E.g. if your MCU has a comparator, or its dedicated brown-out detector has a configurable threshold, and the MCU can run at lower voltages than your fingerprint sensor can. Then it's just a matter of wiring up the interrupt and controlling a MOSFET with a spare GPIO to turn off the sensor.
JDW:
--- Quote from: mark03 on August 28, 2019, 03:34:41 am ---I thought you said you were using an ARM.
--- End quote ---
An ARM is inside the fingerprint sensor module which is made by another company, yes. And to see a photo and description of the bottom of that sensor, please see my previous post and click on the graphic I attached.
Note that the manufacturer of the fingerprint sensor told me the following:
When the module's power supply (powered by battery) is between 2.0v and 2.5v, the memory might be erased. We suggest you add a low voltage detection circuit. For example, when Vcc falls below 2.7V, power to the sensor would be cut.
My MCU is a PIC16F1508 and has a Brown-out Voltage Reset feature and selectable threshold voltage of:
Min. 2.55v, Typ. 2.70v, Max. 2.85v
Since 2.55v is higher than the 2.50v the fingerprint sensor manufacturer mentioned, that brown-out range would qualify BUT I am using that to keep my PIC in RESET. And while my PIC is in RESET, I cannot trigger interrupts or do anything with the PIC at all.
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