Reset & Oscillator Circuit of 8051 Micro-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.

There are two method of reset circuit: 

  

1.Power On Reset.

•     Initially charging of capacitor makes RST High
•     When capacitor charges fully it blocks DC.

2.Manual Reset

•    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

• The 8051 uses the crystal for precisely that to synchronize it’s operation. Effectively, the 8051 operates using what are called "machine cycles."
• A single machine cycle is the minimum amount of time in which a single 8051 instruction can be executed. although many instructions take multiple cycles.  
• 8051 has an on-chip oscillator. It needs an external crystal that decides the operating frequency of the 8051. 

 

 

This can be achieved in two ways.

• 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 oscillator can also be a TTL clock source connected with a NOT gate as shown.