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micro controller 89C51

l AT89C51_1

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 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) pinsLow-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


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