22-03-2012, 12:14 PM
8051 PIN OUT
8051 PIN OUT.doc (Size: 29.5 KB / Downloads: 37)
EA/VP Pin:
Tied to Vcc when external memory is not used.
Tied to Ground when external memory is used.
Reset Circuit:
RESET is an active High input.The 8051 is reset by holding the RST high for at least two machine cycles and then returning it low.
Power-On Reset:
- Initially charging of capacitor makes RST High
- When capacitor charges fully it blocks DC and High from Reset pin goes to ground.
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.
Oscillator Circuit:
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.
8051 has an on-chip oscillator. It needs an external crystal thats 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. 11.059MHz crystal is often used and the capacitance ranges from 20pF to 40pF. This value is used when baud rate which is bits per second of Microcontroller need to be synchronize with PC Baud rate.
The oscillators can also be a from external clock source like functional generator.
How fast 8051 works:
A cycle is, in reality, 12 pulses of the crystal. 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