Reset Circuit of 8051 Controller

RESET CIRCUIT OF 8051 CONTROLLER  RESET is an active High input .  When RESET is set to High, 8051 goes back to the power on state.

 The 8051 is reset by holding the RST high for at least two machine cycles and then returning it low.  The capacitor is to cause a proper reset when power is first applied. When power is applied, the capacitor begins to charge and holds the reset line high (until it is charged) for long enough to ensure that the 8051 gets a proper start up condition.  This circuit ensures that a system or a device starts its operation when supply voltage stabilizes reaches it's nominal value .  the capacitor provides a very short delay for the power supply to steady at 5 volts before the 8051 starts running.  It is needed to prevent erratic or uncontrolled operation during transition period after switching on.  During power off, is to sense when the voltage supply is turned off.

Power On Reset 

Initially charging of capacitor makes RST High.

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When capacitor charges fully it blocks DC. Manual Reset

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closing the switch momentarily will make RST High.

After a reset, the program counter is loaded with 0000H but the content of on-chip RAM is not affected.

Note: content of on-chip RAM is not affected by Reset.

OSCILLATOR CIRCUIT OF 8051 CONTROLLER Crystal oscillator is an electronic oscillator used to create an electrical signal of precise frequency by using the vibrating crystal mechanical resonance made of piezoelectric material.

The crystal is connected to pins 18 and 19 with stabilizing capacitors. 12 MHz (11.059MHz) crystal is often used and the capacitance ranges from 20pF to 40pF.  The PIC will internally divide the clock input frequency by 4, in order to execute the

program instructions.  4 pulses = 1 operation, 1 operation = 1 instruction execution?  Microcontrollers need to use a clock because they need to be able to respond to events that may occur at any time, including nearly simultaneously with either other external events or events generated by the controllers themselves, and will often have multiple circuits that need to know whether one event X precedes another event Y  

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How fast 8051 works ? A cycle is, 12 pulses of the crystal. That is to say, if an instruction takes one machine cycle to execute, it will take 12 pulses of the crystal to execute. Since we know the crystal is pulsing 11,059,000 times per second and that one machine cycle is 12 pulses, we can calculate how many instruction cycles the 8051 can execute per second: 11,059,000 / 12 = 921,583