PIC Microcontroller - PIC16F877A

What Is A Microcontroller

A microcontroller is an all in one electronic integrated circuit with built in processor memory and peripherals that is also re-programmable.

You may not be able to recognize the microcontroller as for cost savings its cheaper to build an ASIC (Application Specific Integrated Circuit). This is a custom chip designed only for the product which is mass produced in milliions making the cost of the device far cheaper than using an individual microcontroller.

Nevertheless each system is based on a microcontroller.

Electronic Circuit design has progressed from individual transistors which are based on silicon to placing thousands and then millions of transistors on a single silicon wafer. The transistors, arranged in the correct configuration, perform different functions such as time delays, boolean logic, memory etc.

7400 series logic

Still available today are the older 74 series devices. These are ICs encased in plastic with connection pins on the outside and each one performs a different well defined function.

By grouping these devices together you can make up circuits by connecting the pins of the device to pins of other devices. About 20 years ago this was the only way of making digital electronic systems and you would typically have racks of circuit boards each with 50 of these devices wired up.

More transistors

Of course the disadvantage of these racks of circuit boards is that once you have designed the board you can not change it so you had to be absolutely sure that the design was perfect. If it was not then you had to start over - and for a complex system that means spending money on a new circuit board and spending time re-designing it.

With the advance of manufacturing techniques more transistors can be put on the silicon wafer - and if you can get more transistors on a wafer there are two advantages.


Increased functionalty.
Higher speed.

Smaller transistors means smaller physical problems (capacitance slows down a devcie - so a smaller device has less) and they also use less current. Less current means less power and less power means less heat and less heat means that the device can operate faster.

Given enough transistors you can design a circuit (or Integrated Circuit - ICs) to perform virtually any function you can think and using a special form of transistor (flash memory) means that the device can store information for years - this memory is also re-programmable.

Processor

The next stage in development was creating the processor which is a specialised circuit in which the function of the device is not set at all. Instead the device is capable of performing different functions when it is given different inputs and not only that - it has a small amount of ram inside used for internal temporary storage (registers) - and this lets it remember a sequence of events.

When you combine this device with two types external memory (other ICs) you can make a system that can follow a defined sequence of events. By reading instructions from the memory (ROM) the processor knows what it should be doing as its next event sequence and it can store data in external RAM memory.


ROM - Read Only Memory (permament)
RAM - Random access memory (temporary - when power goes info is lost).

In addition the processor can read the state of input pins and set the state of output pins so it can react to the outside world e.g. turn on a light or read a key press.

You can see where this is going - it's a device that has re-programmable memory and has many of the building blocks (7400 series) although now greatly refined and adapted inside the device - it's the microcontroller.

Microcontrollers

A microcontroller is a specialized integrated circuit which combines a processor and external memory inside the silicon wafer itself. You now have a customizable device that you can change its function just by re-programming it.

In addition as microcontrollers have been refiined you will find devices that have many other peripherals inside them. Where once you had to buy a UART IC (an RS232 communication device - to talk to a PC) and place it on the processor bus in addition to the RAM and the ROM; this device is now part of the microcontroller itself.

Here are the internal peripherals of the 16F88 a modern PIC microcontroller - this is just an example there are many more microcontroller manufacturers e.g. Atmel, Motorola, Zilog.


RAM 386 Bytes.
ROM 4096 words.
EEPROM 256 Bytes.
I/O 16 pins.
Internal oscillator.
Timer 1.
Timer 2.
Timer 3.
USART. SSP - Synchronous Serial port I2C, SPI protocols.
CCP - Capture Compare PWM.
Analogue comparator 1.
Analogue comparator 2.
ADC multiplexed inputs from 7 pins.

This an 18 pin IC which is as big as some of the original 7400 devices but it can do much more that those single chips. For instance you can make the following types of projects using it:

Universal Infrared decoder/encoder.
Ultrasonic tape measure.
Frequency counter.
Metal detector.

PIC Microcontroller - PIC16F877A 

 The PIC16F877A CMOS FLASH-based 8-bit microcontroller is upward compatible with the PIC16C5x, PIC12Cxxx and PIC16C7x devices. It features 200 ns instruction execution, 256 bytes of EEPROM data memory, self programming, an ICD, 2 Comparators, 8 channels of 10-bit Analog-to-Digital (A/D) converter, 2 capture/compare/PWM functions, a synchronous serial port that can be configured as either 3-wire SPI or 2-wire I2C bus, a USART, and a Parallel Slave Port.

Microchip PIC16F877A Microcontroller Features

High-Performance RISC CPULead-free; RoHS-compliantOperating speed: 20 MHz, 200 ns instruction cycleOperating voltage: 4.0-5.5VIndustrial temperature range (-40° to +85°C)15 Interrupt Sources35 single-word instructionsAll single-cycle instructions except for program branches (two-cycle)Special Microcontroller FeaturesFlash Memory: 14.3 Kbytes (8192 words)Data SRAM: 368 bytesData EEPROM: 256 bytesSelf-reprogrammable under software controlIn-Circuit Serial Programming via two pins (5V)Watchdog Timer with on-chip RC oscillatorProgrammable code protectionPower-saving Sleep modeSelectable oscillator optionsIn-Circuit Debug via two pins

Peripheral Features

33 I/O pins; 5 I/O portsTimer0: 8-bit timer/counter with 8-bit prescalerTimer1: 16-bit timer/counter with prescalerCan be incremented during Sleep via external crystal/clockTimer2: 8-bit timer/counter with 8-bit period register, prescaler and postscalerTwo Capture, Compare, PWM modules16-bit Capture input; max resolution 12.5 ns16-bit Compare; max resolution 200 ns10-bit PWMSynchronous Serial Port with two modes:SPI MasterI2C Master and SlaveUSART/SCI with 9-bit address detectionParallel Slave Port (PSP)8 bits wide with external RD, WR and CS controlsBrown-out detection circuitry for Brown-Out Reset

Analog Features10-bit, 8-channel A/D ConverterBrown-Out ResetAnalog Comparator module2 analog comparatorsProgrammable on-chip voltage reference moduleProgrammable input multiplexing from device inputs and internal VREFComparator outputs are externally accessible

How to handle interrupts in PIC microcontroller ?

Many times we have to write the code for handling interrupts in PIC microcontrollers (e-g when you need to control servo motor which has position encoder, with your PIC microcontroller). This post is a guide on this issue. I am going to explain interrupt handling with the example of 'Capturing external interrupts on RB0/INT pin'. You can download complete code and simulation in Proteus from the 'Downloads' section in the end of this post. The following circuit diagram made on Proteus7.6 shows the setup used to capture interrupt on RB0/INT pin.

Figure 1.   Posedge Interrupt Capturing on PIC16F877

In the figure above push button was pressed five times, that is why pin 19 and pin 21 are high ( PORTD displays how many interrupts have been captured). Whenever push button is pressed then an interrupt is triggered on the posedge of the signal (on pin RB0/INT). This interrupt is processed and PORTD output is incremented by one.

Here is the code to enable external posedge triggered interrupts on the RB0/INT pin.

Figure 2.   InitExternal_INT function code

You will need to call InitExternal_INT() from your main. When the interrupt occurs then ISR is executed, which is shown below.

 

Figure 3.   ISR code for external interrupt handling in PIC

Many PIC microcontrollers have the same interrupt handling function defined with the key word 'interrupt' for all the interrupt sources. That is why it is necessary to check the particular interrupt flag in the ISR to find out the actual source of the interrupt. For example in the code above a function named 'ISR' (stands for Interrupt Service Routine) is defined to handle all the interrupts. In this function firstly INTF flag is checked, (which is one if the external interrupt was triggered) if external interrupt was trigerred then PORTD value is incremented by one and interrupt is cleared.

The main function code is shown below.

Figure 4.   Main function for external interrupt capturing

Main function just enables the external interrupts on RB0/INT pin and then PORTD value is cleared to zero after setting PORTD direction to be output. After that main function just sits in the while loop for ever.

Using the code given in this post you will be able to easily handle interrupts in the PIC microcontroller. As an example, when you need to control servo motor (which has position encoder), then you can attach the output of position encoder on your PIC RB0/INT pin and capture interrupts on it. Whenever servo motor moves one position, then you will receive one interrupt. By incrementing a variable on each interrupt you can always keep track of servo motor position in real time.

You can leave your comments in the comment section below.

Downloads

The code was compiled in MPLAB v8.76 with HI-TECH C and simulation was made in Proteus v7.6.
To download code click here.
To download proteus simulation click here.