Reset Circuit of 8051 Microcontroller

  • Reset occurs when the RST pin is supplied with a positive pulse in duration of at least 2 machine cycles (24 clock cycles of crystal oscillator).
  • After that, the microcontroller generates an internal reset signal which clears all SFRs, except SBUF registers, Stack Pointer and ports (the state of the first two ports is not defined, while FF value is written to the ports configuring all their pins as inputs).
reset values of registers in 8051
  • Depending on surrounding and purpose of device, the RST pin is usually connected to a power-on reset push button or circuit or to both of them.
  • Let’s see reset circuits used for microcontroller 8051. They are of two types.
    1. Power on reset
    2. Manual reset

Power on Reset in 8051

Figure (1) illustrates one of the simplest circuit providing safe power on reset in 8051 microcontroller.

Reset Circuit of 8051 Microcontroller
  • After turning the power on, electrical capacitor is being charged for several milliseconds through a resistor connected to the ground. The pin is driven high during this process.
  • When the capacitor is charged, power supply voltage is already stable and the pin remains connected to the ground, thus providing normal operation of the microcontroller.

Manual Reset in 8051

  • A reset occurs when the capacitor is shorted through the switch which causes the capacitor to be temporarily discharged and the microcontroller is reset.
  • When the switch is released, the capacitor begins to charge, once again pull the reset pin low for normal operation. The reset circuit with the button is shown in figure (2).

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