Mumbai University > Electronics Engineering > Sem 6 > Embedded System and RTOS

Brown Out detection

• Mostly all microcontrollers have built in Brown-out detection (BOD) circuit, which monitors supply voltage level during operation.
• BOD circuit is nothing more than comparator, which compares supply voltage to a fixed trigger level.
• If microcontroller doesn’t have On-Chip Brown-Out detector, then there can be external circuit used.

• The circuit above illustrates the brown out detectors circuit.
• In real word there are special IC where additional delay circuitry and hysteresis used because normalizing of supply voltage may take some time.
• Such IC’s are more cheap than one built from discrete components.
• As an example lets take Atmega128 microcontroller which have On-chip Brown-Out detector. Trigger level can be 2,7V or 4,0V (selected by programming Fuse bits). Trigger level also has a hysteresis to avoid spikes. The hysteresis can be interpreted as VBOT+=VBOT+VHYST/2 and VBOT-=VBOT-VHYST/2.

• So if Brown-Out circuit is enabled by the Fuse bit and Vcc voltage decreases to a value bellow trigger value (VBOT-), Then Brown-Out reset is activated and after supply voltage increases above triggered level (VBOT+) then delay timer starts counting for keeping MCU on reset for some time. After timer counts up, then internal reset signal finishes.