Specifications

mikroElektronika | Free Online Book | PIC Microcontrollers | Chapter 8: Other MCU's Circuits
When power supply voltage reaches 1.2 - 1.7V, a circuit
called Power-up timer resets the microcontroller within
approximately 72mS. Immediately upon this time has run
out, the reset signal generates another timer called
Oscillator start-up timer within 1024 quartz oscillator
periods. When this delay is over (marked as T reset in
figure) and the MCLR pin is set high, the microcontroller
starts to execute the first instruction in the program.
Fig. 8-19 Oscillator Start-Up Time Delay
Apart from such- “controlled” reset which occurs at the moment power goes on, there are another two resets called Black-
out and Brown-out which may occur during operation as well as at the moment power goes off.
Black-out reset
Black-out reset takes place when the power supply normally goes
off. In that case, the microcontroller has no time to do anything
unpredictable simply because the voltage drops very fast beneath
its minimal value. In other words- the light goes off, curtain falls
down and the show is over!
Fig. 8-20 Black-Out Reset at Loss Of Power
Brown-out reset
When power supply voltage drops slowly (typical example of that
is battery discharge although the microcontroller experiences far
faster voltage drop as a slow process), the internal electronics
gradually stops operating and so called Brown-out reset occurs.
In that case, prior to the microcontroller stops operating there is
a serious danger that circuits which operate at higher voltages
start perform unpredictable. It can also causes fatal changes in
the program itself because it is saved in on-chip flash memory.
Fig. 8-21 Brown-Out Reset at Gradual Loss Of Power
Noises
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