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Monday, March 10, 2014

Brownout Protection

Well almost all microcontrollers have Brownout protection inbuilt in them but when connecting a controller to an industry sensor and controlling devices(which are extremely costly) its better we know what is a brownout and how is it detected in a microcontroller cause many devices in low to medium scale industry may not be as immune to brownout as our controller.

So whats a brown out??

Wikipedia defines Brownout as an intentional or unintentional drop in voltage in an electrical power supply system. Intentional brownouts are used for load reduction in an emergency. The reduction lasts for minutes or hours, as opposed to short-term voltage sag or dip.
The brown out can cause one of the three things for a dc supply system. These things in turn can damage the connected embedded systems.

  1. An unregulated direct current supply will produce a lower output voltage for electronic circuits. The output ripple voltage will decrease in line with the usually reduced load current.
  2. A linear direct current regulated supply will maintain the output voltage unless the brownout is severe and the input voltage drops below the drop out voltage for the regulator, at which point the output voltage will fall and high levels of ripple from the rectifier/reservoir capacitor will appear on the output.
  3. A switched-mode power supply which has a regulated output will be affected. As the input voltage falls, the current draw will increase to maintain the same output voltage and current, until such a point that the power supply malfunctions.

Brownouts can cause unexpected behaviour in systems with digital control circuits. Reduced voltages can bring control signals below the threshold at which logic circuits can reliably detect which state is being represented. As the voltage returns to normal levels the logic can latch at an incorrect state; even can't happen states become possible. The seriousness of this effect and whether steps need to be taken by the designer to prevent it depends on the nature of the equipment being controlled; for instance a brownout may cause a motor to begin running backwards.


Working of a Brown Out Detector in a Microcontroller.

Brown out detector is nothing more than a comparator. It basically compares supply voltage with a fixed triggered voltage level during operation. So in a microcontroller the BOD comtinuously compares supply voltage so as to trigger a reset of controller if supply voltage fluctuates below a triggered voltage level. When triggered it immediately cuts of the supply to peripheries and memories while freezing the status of controller and doing a reset. All types of memories like SRAM and EEPROM can get corrupted due to effect of a brown-out except flash program memories. Why?? cause most of the controllers cannot write to their own Flash program memories.

A simple Brown out Circuit can be seen below
external brown out circuit
Brown Out Circuit

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