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Dahlan Rp Sitompul & Poltak Sihombing
A 8051 Microprocessor will be in a reset state if pin 9 (RESET pin) see figure 1 above gets a 5 V DC voltage (Active High) for at least 2 MCs (MC-Machine Cycle)[4][15][19].
Figure 2: MC-Machine Cycle [4] Figure 2 above shows a machine cycle, it can be seen from the figure that a machine cycle consists of six states (State 1-State 6) and each state consists of two pulses (P1 and P2), each pulse (P) is a full cycle of the oscillator (clock)[4]. In other words, a machine cycle consists of 12 pulses of oscillator (clock); a single machine cycle is the minimum time required to perform an instruction of 8051 microcontroller[15][16], several instructions require more than one machine cycle (2 and 4) to complete them[16][17]. In general an instruction of 8051 microcontroller requires only one machine cycle (12 clock pulses), but some instructions require 2 Machine Cycles and two instructions take 4 Machine Cycles (MUL AB and DIV AB)
[17]
to finish their work
[16][17]
. From the above discussion we can see that to put 8051
microprocessor in a valid reset state we must connect pin 9 of the 8051 microprocessor to a 5 Volt DC voltage source for at least 24 oscillator cycles (P) or 2 Machine Cycles, 2 X 12 oscillator cycles
[4][13]
. The period of the oscillator cycle (P)
depends on the type of crystal used in the oscillator circuits. A 12 MHz crystal is generally used, but for application in associated with serial data communications a 11.0592 MHz crystal should be used[4][15]. In this study we will use the 12 MHz crystal oscillator; you can do the same calculation for a 11.0592 MHz crystal oscillator as for 12 MHz crystal oscillator. For crystal oscillator with a frequency of 12 MHz (12 x 106 Hz) there will be 12 million (12 X 106) pulses per second. Thus the period (T) of one pulse is
seconds (1 μ sec); 12 X
1 X 10-6 s (seconds), therefore the period of one machine cycle is 10-6 12
1 X 10-6seconds. As it has been mentioned before in order to enter the valid reset state, the 8051 12
microcontroller pin 9 (reset) has to be connected to 5 volts (Vcc) for > 2 μ sec (2 Machine Cycles)[4] . This term of time > 2 μ sec (2 Machine Cycles) will be used by the author as one of the requirements for calculating the appropriate value of R and C of 8051 microcontroller reset circuits. There are two types of reset of the 8051 microprocessor, known as power on reset and manual reset see figure 1
[2][15]
. Power on reset occurs when microprocessor gets either a DC voltage of 5 Volt from a battery or other source
of DC voltage; voltage source with a regulator that can maintain a 5 V (DC) output voltage such as the LM7805 [9]. While the manual reset, reset will occur when the reset button is pressed [4]. Impact Factor (JCC): 4.9467
Index Copernicus Value (ICV): 3.0