DIGITAL COMMUNICATION USING THE PIC16F84A MICROCONTROLLER

DIGITAL COMMUNICATION USING THE PIC16F84AMICROCONTROLLERMelonee WisePhysics344Fall 2003

Physics 344 Final Project Fall 2003Digital Communication By Melonee WisePhysics Department, University of IllinoisInstructor: Steve ErredeTABLE OF CONTENTSPURPOSE . 3A PROTOTYPE ROBOT . 4Alpha Zippy. 4Tether Controlled H-bridge Circuit. 5COMMUNICATION HARDWARE AND METHODS . 61. PIC Setup. 6Project Window. 62. Initial Testing . 7LED Flasher Code. 7Simple LED Flasher Circuit . 83. Interrupts. 8Interrupt Code . 94. PWM (Pulse Width Modulation). 10PWM Code. 10PWM Test Circuit . 125. Sending and Receiving Data . 13Send Code . 13Receiver Code. 15Sender/Receiver Circuit . 186. RF . 19Transmitter Circuit . 19Receiver Circuit . 20THE TEST DRIVE . 20THE CREATOR . 21Melonee and Zippy. 21Page 2/21

Physics 344 Final Project Fall 2003Digital Communication By Melonee WisePhysics Department, University of IllinoisInstructor: Steve ErredePurposeThe purpose of this project is to develop digital communication using the PIC16F84Amicrocontroller for interface with RF transceivers to control a robot. The specific goals ofthe project are:1. Construct Prototype Robot2. PIC Chip Setup- Obtain necessary software (MPLAB IDE and PICPRO).- Setup new project within MPLAB.- Configure the assembler code properly for the PIC16F84A chip.3. Hardware Design- Design necessary test circuitry to test each software progression.- Design necessary circuitry to interface the PIC chips and the RF moduleswith the prototype robot.4. Software Design- Write a LED flasher program.- Write an interrupt program.- Write a PWM program.- Write a sender program.- Write a receiver program.5. Test the PIC send and receive protocol.6. Test the PIC chip with RF.7. Drive robot around using digital RF controller.Page 3/21

Physics 344 Final Project Fall 2003Digital Communication By Melonee WisePhysics Department, University of IllinoisInstructor: Steve ErredeA Prototype RobotFor this project a prototype robot, Zippy, was built (Figure 1).Figure 1 Alpha ZippyZippy’s main components consist of:2 pieces of plywood2 bicycle training wheels4 bicycle training wheel brackets2 Pitman motors (donated by the ECE department)1 12V motorcycle battery2 pieces of 1 ½ PVC pipe (hold the motors and shafts in place)2 yards of rope2 tape holders (counterbalance the battery)10 or so nuts and bolts2 H-bridges1 receiver1 kill switchOf the hardware used to construct Zippy only the electrical components will bediscussed within this report because it obvious to see the degree of craftsmanshipthat was necessary to construct the prototype.Initially Zippy was a tether controlled robot, a switch controller was linkeddirectly to the speed controller. The speed controller consisted of two H-bridgesthat controlled each motor separately. The N-type and P-type mosfets weresalvaged off of old computer boards and can handle a max of 6 Amps. A 12Vmotor cycle battery was used to power the motors and four, slightly used, 1.5V(double A) batteries were used to power the switch logic. A 3 Amp fuse was usedon the 12V battery to stop potential hazards (i.e. fires, exploding mosfets, ect.).Page 4/21

Physics 344 Final Project Fall 2003Digital Communication By Melonee WisePhysics Department, University of IllinoisInstructor: Steve ErredePNMPNOptical NMPN12V3 AmpPIN1PIN2PIN3PIN4PIN5PIN6PIN7PIN8Figure 2 Tether Controlled H-bridge CircuitPage 5/21

Physics 344 Final Project Fall 2003Digital Communication By Melonee WisePhysics Department, University of IllinoisInstructor: Steve ErredeCommunication Hardware and Methods1. PIC SetupBefore beginning the project, the necessary programs must be obtained. MPLABcan be downloaded for free from http://www.microchip.com and a programmerand programming software can obtained from http://ramseyelectronics.com. Afterthe proper software and equipment are obtained the PIC chip must be correctlyconfigured in MPLAB so that it can be programmed properly and functionsproperly when tested. Remember to always read the PIC data sheet beforebeginning any project.First a project must be setup in MPLAB by creating a new project using theproject wizard. The project wizard will step through selecting the proper device(PIC16F84A), the proper language toolsuite (MPASM Assembler), and finally theproject name and directory. Within the project the proper include and linker filesmust be added, the include files can be found in the MPLAB IDE folder underdisPIC Tools/support/inc and the linker files in the MPLAB IDE folder underMCHIP Tools/Lkr. Finally a main source file must be created for executablecode. When these steps are completed the project window should look similar toFigure 3.Figure 3 Project WindowNext it is important to have the proper configuration; this sets the oscillator type,the watchdog timer, copy protection, and power up timer. For this project, themain source file is configured in the following manor,config WDT OFF & PWRTE ON & HS OSC & CP OFF,Page 6/21

Physics 344 Final Project Fall 2003Digital Communication By Melonee WisePhysics Department, University of IllinoisInstructor: Steve Erredethis turns the watchdog timer off, the power up timer on, sets the oscillator to highspeed, and turns copy protection off.Finally the banks, ports, and interrupts must be configured properly. For thisproject, the bank is set to bank 0, A ports are set as inputs, B ports are set asoutputs, and to begin with all of the interrupts are disabled. The interrupts will beused later in the project but for initial testing and setup they are disabled.2. Initial TestingTo quickly test whether the PIC chip has been configured correctly the followingassembler program and circuit (Figure 4) can be used.LED Flasher Code:#include P16F84A.INC processor 16f84Aconfig WDT OFF & PWRTE ON & HS OSC & CP OFFRESET CODE 0GOTO INITCODEINIT:BCF STATUS, RP1BSF STATUS, RP0CLRF INTCONMOVLW B'11111111'MOVWF TRISAMOVLW B'00000000'MOVWF TRISBMOVLW B'10001000'MOVWF OPTION REGBCF STATUS, RP0START:MOVF PORTA, WMOVWF PORTBCLRWDTGOTO STARTEND;set to bank 1;disable all interrupts;make A ports input;make B ports output;return to bank 0;copy PORTA to PORTBPage 7/21

Physics 344 Final Project Fall 2003Digital Communication By Melonee WisePhysics Department, University of IllinoisInstructor: Steve ErredeFigure 4 Simple LED Flasher Circuit3. InterruptsTiming is very important when trying to send and receive data; interrupts can beused to create the necessary timing. The timer interrupt for the PIC16F84A occursevery 256 instruction cycles. The amount it takes to complete one instructioncycle varies based on the frequency of oscillator input and can be calculated usingequation 1,1 time to complete one instruction . Oscillator frequency 4 (1)The time between interrupts is given by equation 2, 4 Oscillator frequency time between interrupts .256(2)The timer interrupt can be used to call functions at constant intervals by countingthe number of interrupts that occur between calls. Equation 3 can be used tocalculate the number of interrupts that happen in a certain time interval.Page 8/21

Physics 344 Final Project Fall 2003Digital Communication By Melonee WisePhysics Department, University of IllinoisInstructor: Steve Errede Oscillator frequency time 4 number of interrupts in x seconds .256(3)For this project a 16.25MHz clock was used, all calculations are based on thisoscillator frequency.When an interrupt occurs the PCL of the chip is set to register 4 in the memorybank, i.e. the interrupt code must start at 4. When using an interrupt special caremust be taken so that the program returns with all of the correct values in the Wand STATUS registers. For this project interrupts were used as a timer, each timethe interrupt was called it incremented a register which the main programchecked. When the registers reached a certain value/time the send or receivefunction was called to send or receive data. Make sure to use the appropriatenumber of registers for the amount of time needed, each register can only holdtwo bits in HEX.The following interrupt code demonstrates how to store the W and STATUSregisters and increment counter registers for timing.Interrupt Code:#include P16F84A.INC processor 16f84AUDATAW TEMP res 1STATUS TEMP res 1GLOBAL TIMEC1TIMEC1 res 1GLOBAL TIMEC2TIMEC2 res 1INTERRUPT CODE 4GOTO PUSH;sets code start to 4th registerCODEGLOBAL PUSHPUSH:;save W and statusMOVWF W TEMPSWAPF STATUS, WMOVWF STATUS TEMPISR:INCFSZ TIMEC1, FGOTO POPINCF TIMEC2, FPOP:;interrupt service routine;if zero increment next register;if zero increment next register;restore W and statusBCF INTCON, T0IFSWAPF STATUS TEMP, WMOVWF STATUSSWAPF W TEMP, FSWAPF W TEMP, WPage 9/21

Physics 344 Final Project Fall 2003Digital Communication By Melonee WiseRETFIEENDPhysics Department, University of IllinoisInstructor: Steve Errede;return from interrupt4. PWM (Pulse Width Modulation)Pulse width modulation, in its simplest form, takes a binary number and creates apulse of the appropriate width, as shown in Figure 5.001010011Figure 5 PWM examplePWM is extremely useful for motor speed control, if a motor is turned on and offquickly (quick enough to have smooth motion) the motor speed is determined bythe portion of time it is on. The PWM code for this project was mainly used to testthe interrupt code but will be used in a later project to create a variable speedcontroller for the robot.The code and circuitry (Figure 6) shown below can be used to create a verysimple and slow PWM to visually check whether the interrupts have been setupcorrectly. Basically it takes a three bit (0-8) binary number from PORTA and usesit to make a modulated pulse. Each segment of the pulse is one second long, so ifthe input is two then the output would be high for two seconds and low for sixseconds. The code has three parts, a main function, PWM function, and aninterrupt (the interrupt code is shown above).PWM Code:PWM#include P16F84A.INC processor 16f84AUDATAPWMC res 1Page 10/21

Physics 344 Final Project Fall 2003Digital Communication By Melonee WisePhysics Department, University of IllinoisInstructor: Steve ErredeGLOBAL PWMCODEPWM:MOVLW .6;decrement or reset counterDECF PWMC, FBTFSC PWMC, 7MOVWF PWMC;output the low bits of PORTA onto the low bits of PORTBMOVLW B'00000111'ANDWF PORTA, WIORWF PORTB, FMOVLW B'11111000'IORWF PORTA, WANDWF PORTB, FMOVFW PORTA;check count against input valueANDLW B'00000111'SUBWF PWMC, WBTFSS STATUS, CGOTO HIGHOUTGOTO LOWOUTHIGHOUT:BSF PORTB, 6GOTO NEXT;set bit 6 highLOWOUT:BCF PORTB, 6;set bit 6 lowNEXT:RETURNENDMAIN#include P16F84A.INC processor 16f84Aconfig WDT OFF & PWRTE ON & HS OSC & CP OFFEXTERN TIMEC1EXTERN TIMEC2EXTERN PWMRESET CODE 0GOTO INITCODEINIT:BCF STATUS, RP1BSF STATUS, RP0CLRF INTCONMOVLW B'11111111'MOVWF TRISAMOVLW B'00000000'MOVWF TRISBMOVLW B'10001000'MOVWF OPTION REGBCFSTATUS, RP0MOVLW B'10100000'MOVWF INTCONSTART:BCF INTCON, GIEMOVLW H'FD';set to bank 1;disable all interrupts;make A ports input;make B ports output;return to bank 0;enable timer interrupt;checking to see if has been 1s;stop interrupt while checking timePage 11/21

Physics 344 Final Project Fall 2003Digital Communication By Melonee WiseSUBWF TIMEC1, WBTFSS STATUS, ZGOTO NEXTMOVLW H'3D'SUBWF TIMEC2, WBTFSS STATUS, ZGOTO NEXTMOVLW B'10000000'XORWF PORTB, FCLRF TIMEC1CLRF TIMEC2CALL PWMPhysics Department, University of IllinoisInstructor: Steve Errede;clearing out the timers after 1sNEXT:BSF INTCON, GIECLRWDTGOTO STARTEND;turning interrupts back onFigure 6 PWM Test CircuitPage 12/21