This tutorial will introduce you the basics about programming the input and output ports on an 8051 microcontroller using C language. Therefore, it is recommended that the reader is familiar or has basic knowledge about C programming language and electronics circuit analysis. I am going to use Atmel‘s AT89C2051 as an example for the 8051 microcontroller and the C compiler that I am going to use is the RC-51 which is included with the Free Evaluation 8051 Software Toolset of Raisonance. You may see this for more info about the toolset or you may download the free evaluation 8051 Software Toolset here(RKit-Eval51). See this tutorial for a quick start guide with this software.
For an introduction about AT89C2051, see this.
AT89C2051 General Input and Output Ports
The original 8051 microcontroller (40 pins) contains 4 digital input and output ports which are P0, P1, P2, and P3. Its little brother, AT89C2051 (20pins), only contains two bidirectional input and output ports which are P1 and P3. Both ports are one-byte (8-bits) wide and each pin of each port can be accessed externally (see the pin diagram below) except bit 6 of P3 or P3.6.
P3.6, however, do exist. It is hardwired internally to the output of AT89C2051′s on-chip analog comparator (see diagram below). All port pins of AT89C2051 have internal pullups except P1.0 and P1.1. Pins P1.0 and P1.1 do not have internal pullups because these pins are also used as inputs of the on-chip analog comparator.
As you can see, the pins of AT89C2051 has alternate functions and we will discuss those functions in the next series of tutorials about AT89C2051. Just for a quick view of these alternate functions, please look at the table below.
| Port Pin | Alternate Functions |
| P1.0 | AIN0 (positive input) of the on-chip precision analog comparator |
| P1.1 | AIN1 (negative input) of the on-chip precision analog comparator |
| P3.0 | RXD (serial input port) |
| P3.1 | TXD (serial output port) |
| P3.2 | INT0 (external interrupt 0) |
| P3.3 | INT1 (external interrupt 1) |
| P3.4 | T0 (timer 0 external input) |
| P3.5 | T1 (timer 1 external input) |
Hi there! Thank you very much for this great tutorial! I read it with interest and am sure, I am going to use an 8051 for one of my projects. The problem is, I just don’t know how to get the programm I wrote into my microcontroller. Wich kind of programmer do I nead? Thank you!
Ooooo! Nice!BTW…Love the beannr with the jawa on Tatooine (complete with two suns) and the site name. Very impressive.
Suppose i remove the switch SW1 and i put a switch in between 10k resistor and port pin . so when i press the switch current flows to the port (so port has input bit -1) and when not pressed no current flows (port has no input bit – 0 ). how to check this condition.