8th week ( 12 Mar 2012 - 16 Mar 2012 )

Tittle of activity :

• Project research

Microcontroller PIC16F877A

      The microcontroller is simply a computer on a chip. It is one of the most important developments in electronics since the invention of the microprocessor itself. It is essential for the operation of devices such as mobile phones, DVD players, video cameras, and most self-contained electronic systems. The small LCD screen is a good clue to the presence of an MCU (Microcontroller Unit) – it needs a programmed device to control it. Working sometimes with other chips, but often on its own, the MCU provides the key element in the vast range of small, programmed devices which are now commonplace.

 PIC 16F877 Architecture

           Microcontrollers contain all the components required for a processor system in one chip: a CPU, memory and I/O. A complete system can therefore be built using one MCU chip and a few I/O devices such as a keypad, display and other interfacing circuits. We will now see how this is done in practice in our typical microcontroller.

PIC 16F877 Pin Out

      

 Let consider the pins that are seen on the IC package, and then discover how they relate the internal architecture. The chip can be obtained in different packages, such as conventional 40-pin DIP (Dual In LinePackage), square surface mount or socket format. The DIP version is recommended for prototyping, and is shown in Figure below 

       Most of the pins are for input and output, and arranged as 5 ports: A(5), B(8), C(8), D(8) and E(3), giving a total of 32 I/O pins. These can all operate as simple digital I/O pins, but most  have more than one function, and the mode of operation of each is selected by initialising various 

control registers within the chip. Note, in particular, that Ports A and E become ANALOGUE  INPUTS by default (on power up or reset), so they have to set up for digital I/O if required.

      

        Port B is used for downloading the program to the chip flash ROM (RB6 and RB7), and RB0 and RB4–RB7 can generate an interrupt. Port C gives access to timers and serial ports, while Port D can be used as a slave port, with Port E providing the control pins for this function. 

        The chip has two pairs of power pins (VDD 5 V nominal and Vss 0V), and either pair can be used. The chip can actually work down to about 2 V supply, for battery and power-saving operation. A low-frequency clock circuit using only a capacitor and resistor to set the frequency can be connected to CLKIN, or a crystal oscillator circuit can be connected across CLKIN and CLKOUT. MCLR is the reset input; when cleared to 0, the MCU stops, and restarts when MCLR   1. This input must be tied high allowing the chip to run if an external reset circuit is not connected, but it is usually a good idea to incorporate a manual reset button in all but the most trivial applications.