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PIC MICROCONTROLLER APPLICATION BASICS
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inadvertently enabling the watchdog timer. When this happens and you are new to the PIC microcontroller, nding the problem is just about impossible, and as I discussed earlier, the problem can appear to be the same as if the decoupling capacitor was forgotten or incorrectly wired. This means that if you have an application that seems to start okay but resets itself, you probably should take the PIC microcontroller out of the application and check it in your programmer to see if the watchdog timer is enabled. For some programs (such as the ones presented in this book), you will have to go back to the source and make sure that the _WDT_OFF parameter is speci ed and not forgotten.
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TMR0: TMR0 SETUP WITH PRESCALER
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As I work through the later experiments and the projects, I will be using the TMR0 and prescaler hardware a lot. This experiment also will give you an idea of the speeds at which the PIC microcontroller operates. For this experiment, you may want to look at the data output with an oscilloscope, but it is not necessary if you have a logic probe. The TMR0 application (which can be found in the code\TMR0 folder) is another one that uses PORTB for LED outputs. What TMR0 does is enable TMR0 to run in from the processor s clock and then poll the TMR0 value and write it out to PORTB so that the value of TMR0 can be checked externally. The circuitry is shown in Fig. 20.9 and can be built on a breadboard as in Fig. 20.10. The bill of materials is listed in Table 20.5.
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title TMR0 - Demonstrate the Operation of TMR0 with Prescaler ; ; This Code sets up TMR0 to run from the Instruction Clock and ; uses the Prescaler to divide the incoming clock. ; ;
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Vcc 16F84
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0.1 uF Tantalum Gnd
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_MCLR Osc 1 Osc 2 Vss
4 MHz
PORTB
8 Bits
TMR0: displaying TMR0 values on eight LEDs.
SOME BASIC FUNCTIONS
+5 Volt P/S
10 K
Vcc Gnd
The breadboard wiring for the TMR0 application.
; ; ; ; ; ; ; ;
Hardware Notes: PIC16F84 Running at 4 MHz _MCLR is Pulled Up All 8 bits of PortB are Pulled up and Connected to LEDs Myke Predko 99.12.26 LIST R=DEC INCLUDE p16f84.inc
Register Usage
TABLE 20.5 PART
BILL OF MATERIALS FOR THE TMR0 EXPERIMENT DESCRIPTION
PIC microcontroller 4-MHz LED 10-k 220 0.1- F Misc.
PIC16F84 04/P 4-MHz ceramic resonator with internal capacitors 10-LED bargraph display 10 k 220 , 1/4 W , 1/4 W
0.1- F tantalum Breadboard, wiring, +5-V power supply
0.1 uF
PIC16F84
4 MHz
PIC MICROCONTROLLER APPLICATION BASICS
CBLOCK 0x020 ENDC
; Start Registers at End of the Values
PAGE __CONFIG _CP_OFF & _WDT_OFF & _XT_OSC & _PWRTE_ON ; Mainline of TMR0 org 0 nop bsf clrf movlw addlw movwf bcf Loop: comf movwf goto TMR0, w PORTB Loop STATUS, RP0 TRISB ^ 0x080 ; Make All 8 PortB Bits Output 0x0FF ^ ((1 << T0CS) | (1 << PSA) | 7) 0 ; Put in Prescaler Value OPTION_REG ^ 0x080 ; Load the Option Register Value STATUS, RP0 ; ; Loop Here Output the TMR0 Value
The instructions that set up the OPTION register to use the PIC microcontroller s TMR0 clock input and the prescaler for TMR0 will seem a bit cumbersome and hard to understand. Normally, all the bits in the OPTION register are set; to change its operation for specifying TMR0 as using the instruction clock input as well as directing the prescaler to TMR0, the bits have to be reset. In addition, I clear the prescaler s value. To be honest, instead of creating the cumbersome instruction
movlw 0x0FF ^ ((1 << T0CS) | (1 << PSA) | 7)
I simply use the constant value 0x0D7 as
movlw movwf 0x0D7 OPTION_REG ^ 0x080
which is a lot easier to read, and if it is before a movwf OPTION_REG ^ 0x080 instruction, then I know that I am setting up the OPTION register. I used the more complex format to show how the OPTION register bits are set for this application. The next instruction, the addlw 0, is used to set the prescaler to something other than one to one, the reason for which will become obvious.
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