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
