Файл: Basic for PIC Microcontrollers (M. Nebojsa, 2001).pdf

5V voltage is used).

The simplest AD conversion program would use 8-bit resolution and 5V of microcontroller power as referent voltage (value which the value "read" from the microcontroller pin is compared to). In the following example we measure voltage on RA0 pin which is connected to the potentiometer (picture below).

Potentiometer gives 0V in one terminal position and 5V in the other, so that digitalized voltage can take values ranging from 0 to 256 due to the fact that 8-bit conversion is used. The following program reads voltage on RA0 pin and displays it on port B diodes. If not one diode is on, result is zero and if all of diodes are on, result is 255.

At t he very beginning, it is necessary to properly initialize 2 bit registers ADCON1 and ADCON0. Afterwards, only thing required is to set ADCON0.2 bit which initializes the conversion and checks ADCON0.2 to determine if conversion is over. After the conversion is over, result is stored into ADRESH and ADRESL where from it can be copied. Former example can also be carried out via ADCIN instruction. Following example uses 10-bit resolution and ADCIN instruction.

As one port is insufficient, LCD can be used for displaying all of the 10 bits of result. Connection scheme is on the picture below and appropriate program follows.

5.3 Using the TMR0 timer

TMR0 timer is 8 -bit and has working range of 255. Assuming that 4MHz oscillator is used, time period TMR0 can measure falls into 0-256 microseconds range (with 4MHz frequency TMR0 increments by one microsecond). If prescaler is used that period can be

prolonged, because prescaler divides the clock in a certain ratio (prescaler settings are made in OPTION_REG register).

Following program illustrates use of TMR0 timer for generating 1 second time period. Prescaler is set to 32, so that internal clock is divided by 32 and TMR0 increments every 31 microseconds. If TMR0 is initialized on 96, overflow occurs in (256-96)*31 us = 5 ms.

If variable "Brojac" is increased every time interrupt takes place, we can measure time according to the value of variable "Brojac". If "Brojac" is set to 200, time will total 200*5 ms = 1 second.

Before the main program, TMR0 should have interrupt enabled (bit 2) and GIE bit (bit 7) in INTCON register should be set.

5.4 Using the TMR1 timer

Unlike TMR0, TMR1 is 16-bit and has working range of 65536. Assuming that 4MHz oscillator is used, time period TMR1 can

measure falls into 0-65536 microseconds range (with 4MHz frequency TMR01 increments by one microsecond). If prescaler is used that period can be prolonged, because prescaler divides the clock in a certain ratio (prescaler settings are made in T1CON register).

Before the main program, TMR1 should be enabled by setting the zero bit in T1CON register. Besides that, first bit of the register should be set to zero, thus defining the internal clock for TMR1.

Besides T1CON, other important registers for working with TMR1 include PIR1 and PIE1. The first contains overflow flag (zero bit) and the other is used to enable TMR1 interrupt (zero bit).

When TMR1 interrupt is enabled and its flag reset only thing left to do is to enable global interrupts (bit 7) and peripheral interrupts (bit 6) in the INTCON register.

The following program illustrates use of TMR1 register for generating 10 seconds time period. Prescaler is set to 00 so there is no dividing the internal clock and overflow occurs every 65.536 ms. If variable "Brojac" is increased every time interrupt takes place, we can measure one minute period according to the variable "Brojac". If "Brojac" is set to 152, time will total 152*65.536 ms = 9.960 second.

5.5 Using the PWM subsystem

Microcontrollers of PIC16F87X series have one or two PWM outputs built-in (those in 40-pin casing have 2, while those in 28-pin casing have 1). PWM outputs are located on RC1 and RC2 pins in case of 40-pin microcontrollers and on RC2 pin in case of 28-pin microcontrollers. HPWM instruction greatly simplifies using the PWM. There are only 3 parameters to be set :

PWM Channel : defines which PWM channel is used; "1" defines channel on RC1 pin, while "2"

defines channel on RC2 pin.

Ratio_S_P : defines the ratio of on and off signals on pin. "0" defines continual

off state, whereas "255" defines continual on state. All values within these

limits define appropriate ratio of on and off signals on pin. (i.e. "127" gives

50% of 0V on output and 50% of 5V on output).

Frequency : defines PWM signal frequency. Top frequency for any oscillator is 32767Hz.

The following example demonstrates use of PWM for getting various light intensities on LED diode connected to RC1 pin (PWM channel 0). Parameter defining ratio of on and off signals is continually increased in the for-next loop and takes value from 0 to 255, resulting in continual intensifying of light on LED diode. After value of 255 has been reached, process begins anew.

5.6 Using the hardware UART subsystem (RS-232 communication)

Easiest way to transfer data between microcontroller and some other device (i.e. PC or other microcontroller) is the RS-232 communication. It is serial asynchronous 2-line (Tx for transmitting and Rx for receiving) data transfer for within 10m range.

This example shows data transfer between the microcontroller and PC connected by RS-232 line interface (MAX232) which has role of adjusting signal levels on the microcontroller side (it converts RS-232 voltage levels +/- 10V to TTL levels 0-5V and vice versa). Microcontroller can achieve communication with serial RS-232 line via hardware UART (Universal Asynchronous Receiver Transmitter) which is the integral part of PIC16F87X microcontrollers.

UART contains special registers for receiving and transmitting data as well as BAUD RATE generator for determining data transfer rate.

The program below illustrates use of hardware serial communication subsystem (serial communication can also be software based on any of 2 microcontroller pins). Data received from PC is stored into variable B0 and sent back to PC as confirmation of successful transfer. Thus, it is easy to check if communications works properly. Transfer format is 8N1 and transfer rate is 2400 baud.

In order to achieve communication, PC must have the communication software. One such program is part of the MicroCode studio. It can be accessed by clicking View and then Serial Communication Window. New window will appear on screen and can be used for adjusting transfer settings. First it is necessary to set transfer rate by clicking Baudrate on the left of the window (set it to 2400, because microcontroller is set to that rate). Communication port is selected by clicking one of the 4 available depending on port connected to a serial cable.

After making adjustments, clicking Connect starts the communication. Type your message and clickSend Message - message is sent to the microcontroller and back, where it is displayed on the screen.

Chapter 6

SAMPLES WITH PIC16F84 MICROCONTROLLER

Introduction

6.1 LED diode

6.2 Button

6.3 Generating sound

6.4 Potentiometer

6.5 Seven-segment displays

6.6 Step motor

6.7 Input shift register

6.8 Output shift register

6.9 Software serial communication

6.10 Building light control

Introduction

This chapter gives detailed examples of connecting PIC16F84 microcontroller to peripheral components and appropriate programs written in BASIC. All of the examples contain electrical connection scheme and program with comments and clarifications. You have the permission to directly copy these examples from the book or download them from the web site http://www.mikroelektronika.co.yu/ .

6.1 LED diode

One of the most frequently used components in electronics is surely the LED diode (LED stands for Light Emitting Diode). Some of common LED diode features include : size, shape, color, working voltage (Diode voltage) Ud and electric current Id. LED diode can have round, rectangular or triangular shape, although manufacturers of these components can produce any needed shape by order. Size i.e. diameter of round LED diodes ranges from 3 to 12 mm, with 3 or 5 mm sizes most commonly used. Color of emitting light can be red, yellow, green, orange, blue, etc. Working voltage i.e. necessary for LED diode to emit light is 1.7V for red, 2.1V for green and 2.3 for orange color. This voltage can be higher depending on the manufacturer. Normal current Id through diode is 10 mA, while maximal current reaches 25 mA. High current consumption can present problem to devices with battery power supply, so in that case low current LED diode (Id ~ 1-2 mA) should be used. For LED diode to emit light with maximum capacity, it is necessary to connect it properly or it might get damaged.

The positive pole is connected to anode, while ground is connected to cathode. For matter of differentiating the two, cathode is marked by mark on casing and shorter pin. Diode will emit light only if current flows from anode to cathode; in the other case there will be no current. Resistor is added serial to LED diode, limiting the maximal current through diode and protecting it from damage. Resistor

value can be calculated from the equation on the picture above, where Ur represents voltage on resistor. For +5V power supply and 10 mA current resistor used should have value of 330¿.

LED diode can be connected to microcontroller in two ways. One way is to have microcontroller "turning on" LED diode with logical one and the other way is with logical zero. The first way is not so frequent (which doesn't mean it doesn't have applications) because it requires the microcontroller to be diode current source. The second way works with higher current LED diodes.

The following example uses instructions High, Low and Pause to turn on and off LED diode connected to seventh bit of port B every half second.

6.2 Button

Button is a mechanical component which connects or disconnects two points A and B over its contacts. By function, button contacts can be normally open or normally closed.

Pressing the button with normally open contact connects t he points A and B, while pressing the button with normally closed contact disconnects A and B.

Buttons can be connected to the microcontroller in one of two ways:

In the first case, button is connected in a way that logical one (+5V) remains on microcontroller input pin while button is not pressed. Resistor between a button and power voltage has role of holding the input pin in defined state when the button is not pressed (in this case a logical one). This is necessary as a protection from glitch on input pin that might cause misinterpretation of program, i.e. as if button is pressed when it is not.

When the button is pressed, input pin is short circuited to the ground (0V) which indicates change on input pin. Voltage has dropped

from 5V to 0V. This change is interpreted by program as if button was pressed and part of program code tied to a button (for example turn on LED diode) is then executed. This way of defining pin states is called defining with "pull-up" resistors, associating that the line is held up on the logical one level.

In the other case, button is connected in a way that logical zero remains on input pin. Now, resistor is between input pin and a logical zero, meaning that pressing the button brings logical one to input pin. Voltage goes up from 0V to +5V. Microcontroller program should recognize change on input pin and execute the specific part of program code. This way of defining pin states is called defin ing with "pull -down" resistors, associating that the line is held down on the logical zero level.

Common way to connect the button is with pull-up resistors, meaning that pressing the button changes pin state from logical one to logical zero. Following picture displays four button connected to the microcontroller using the pull -up resistors.

Problem that occurs when working with buttons is contact debounce in the moment when button is pressed. Debounce is consequence of the contact and heavily depends on the very button.

One of the ways to solve the contact debounce problem is given in the following part of program code :

Pressing the Button0 causes the program to jump to address Wait0 where it remains in the loop until the button is released (this achieves that single button push is just once handled in program). When Button0 is released program continues executing instructions (in this case variable W is increased by one). Pressing Button1 causes the same effect, except that variable W is decreased by one.

Problem might arise if an interrupt or some other source slows down the program execution, so that program finds itself on Wait0 or

Wait1 lines after the button is released. This might cause program blocking until button is pressed again.

In the following program for reading the button states, BASIC instruction Button is used which eliminates the contact debounce.

The program reads buttons T0 and T1 which are connected to the pins RA0 and RA1, respectively. Pressing the button 0 executes part of program code which turns on LED diode on pin RB0. Pressing the button 1 executes part of program code which turns off LED diode on the same pin. The mentioned instruction is among the most complex instructions of BASIC program language. Besides few arguments that should be defined, instruction has an argument for setting the delay time between recognition of two different button pressures (the third argument). Its setting depends on the purpose of the button as well as mechanical properties of the button. Still, it came clear over time that maximal value of last argument represents the best solution for most applications, because of great disproportion in human reaction and microcontroller speed.