How to Calculate the Capacitor of the Reset Input of a 8051 Microcontroller

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By Jackson Taylor

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

(FROM ATMEL)
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

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