Capacitor

Capacitor is a passive component used to store charge. The charge (q) stored in a capacitor is the product of its capacitance (C) value and the voltage (V) applied to it. Capacitors offer infinite reactance to zero frequency so they are used for blocking DC components or bypassing the AC signals. The capacitor undergoes through a recursive cycle of charging and discharging in AC circuits where the voltage and current across it depends on the RC time constant. For this reason, capacitors are used for smoothing power supply variations. Other uses include, coupling the various stages of audio system, tuning in radio circuits etc. These are used to store energy like in a camera flash.

 

Capacitors may be non-polarized/polarized and fixed/variable. Electrolytic capacitors are polarized while ceramic and paper capacitors are examples of non polarized capacitors. Since capacitors store charge, they must be carefully discharged before troubleshooting the circuits. The maximum voltage rating of the capacitors used must always be greater than the supply voltage.

AT89C51 Microcontroller

http://www.atmel.com/images/doc0265.pdf

AT89C51 is an 8-bit microcontroller and belongs to Atmel's 8051 family. ATMEL 89C51 has 4KB of Flash programmable and erasable read only memory (PEROM) and 128 bytes of RAM. It can be erased and program

to a maximum of 1000 times.

In 40 pin AT89C51, there are four ports designated as P₁, P₂, P₃ and P₀. All these ports are 8-bit bi-directional ports, i.e., they can be used as both input and output ports. Except P₀ which needs external pull-ups, rest of the ports have internal pull-ups. When 1s are written to these port pins, they are pulled high by the internal pull-ups and can be used as inputs. These ports are also bit addressable and so their bits can also be accessed individually.

 

Port P₀ and P₂ are also used to provide low byte and high byte addresses, respectively, when connected to an external memory. Port 3 has multiplexed pins for special functions like serial communication, hardware interrupts, timer inputs and read/write operation from external memory. AT89C51 has an inbuilt UART for serial communication. It can be programmed to operate at different baud rates. Including two timers & hardware interrupts, it has a total of six interrupts.

Pin Diagram : 

Pin Description: 

Pin No	Function			Name
1	8 bit input/output port (P1) pins			P1.0
2				P1.1
3				P1.2
4				P1.3
5				P1.4
6				P1.5
7				P1.6
8				P1.7
9	Reset pin; Active high			Reset
10	Input (receiver) for serial communication	RxD	8 bit input/output port (P3) pins	P3.0
11	Output (transmitter) for serial communication	TxD		P3.1
12	External interrupt 1	Int0		P3.2
13	External interrupt 2	Int1		P3.3
14	Timer1 external input	T0		P3.4
15	Timer2 external input	T1		P3.5
16	Write to external data memory	Write		P3.6
17	Read from external data memory	Read		P3.7
18	Quartz crystal oscillator (up to 24 MHz)			Crystal 2
19				Crystal 1
20	Ground (0V)			Ground
21	8 bit input/output port (P2) pins / High-order address bits when interfacing with external memory			P2.0/ A8
22				P2.1/ A9
23				P2.2/ A10
24				P2.3/ A11
25				P2.4/ A12
26				P2.5/ A13
27				P2.6/ A14
28				P2.7/ A15
29	Program store enable; Read from external program memory			PSEN
30	Address Latch Enable			ALE
	Program pulse input during Flash programming			Prog
31	External Access Enable;  Vcc for internal program executions			EA
	Programming enable voltage; 12V (during Flash programming)			Vpp
32	8 bit input/output port (P0) pins   Low-order address bits when interfacing with external memory			P0.7/ AD7
33				P0.6/ AD6
34				P0.5/ AD5
35				P0.4/ AD4
36				P0.3/ AD3
37				P0.2/ AD2
38				P0.1/ AD1
39				P0.0/ AD0
40	Supply voltage; 5V (up to 6.6V)			Vcc