The reset is used to start-up or to restart the 80S51 microcontroller activities. It forces the 80S51 in a known state by reinitializing all the internal registers needed to properly start the program execution. The reset must be kept active until all three of the following conditions are respected:
- The power supply must be in the specified range.
- The oscillator must reach a minimum oscillation level to ensure a good noise to signal ratio and a correct internal duty cycle generation.
- The reset pulse width duration must be at least two machine cycles. If one of the conditions is not respected the microcontroller will not startup properly
CAPACITOR VALUE DESIGNING
RESET TIME = TSUPPLY RISE TIME +TOSCILLATOR SET TIME
TSUPPLY RISE TIME =1 TO 50mS .Let the time be 5mS
TOSCILLATOR SET TIME = 1 TO 50mS .Let the time be 5mS
We know for RC circuit
TIME CONSTANT ,T= RC
LET PULL DOWN RESISTOR BE 10K
5ms + 5ms= 10K * C
C = 1 uF