PIC18F4520 Timer.

○ PIC 타이머

- PIC18F4520는 3개의 타이머를 가짐.

TIMER0	8-bit	OPTION_REG	TMR0	0.2usec	13.107ms
TIMER1	16-bit	T1CON	TMR1H,TMR1L	0.2usec	104.857ms
TIMER2	8-bit	T2CON	TMR2	0.2usec	819usec

딜레이에 따른 타이머 계산

Delay = TimerCount * tick
Count = (Delay/tick)
RegValue = TimerMax- Count
RegValue = TimerMax-(Delay/tick) = TimerMax - (Delay/((Prescalar *4)/Fosc))
RegValue = TimerMax-((Delay * Fosc)/(Prescalar*4))

TIMER0	8-bit	RegValue = 256-((Delay * Fosc)/(Prescalar*4))
TIMER1	16-bit	RegValue = 65536-((Delay * Fosc)/(Prescalar*4))
TIMER2	8-bit	RegValue = 256-((Delay * Fosc)/(Prescalar*4))

○ Timer 0

• 8-bit timer/counter
• Readable and writable
• 8-bit software programmable prescaler
• Internal or external clock select
• Interrupt on overflow from FFh to 00h
• Edge select for external clock

○ Timer0 레지스터

REG	This registers is used to configure the TIMER0 Prescalar, Clock Source etc
TMR0	This register holds the timer count value which will be incremented depending on prescalar configuration
INTCON	This register contains the Timer0 overflow flag(TMR0IF) and corresponding Inetrrupt Enable flag(TMR0IE).

 

7	6	5	4	3	2	1	0
RBPU	INTEDG	T0CS	T0SE	PSA	PS2	PS1	PS0

RBPU: NA for Timers

INTEDG: NA for Timers

T0CS: TMR0 Clock Source Select bit
1-Transition on T0CKI pin
0-Internal instruction cycle clock (CLKO)

T0SE: TMR0 Source Edge Select bit
1-Increment on high-to-low transition on T0CKI pin
0-Increment on low-to-high transition on T0CKI pin

PSA: Prescaler Assignment bit
1-Prescaler is assigned to the WDT
0-Prescaler is assigned to the Timer0

PS2:PS0: Prescaler Rate Select bits

 

INTCON
7	6	5	4	3	2	1	0
GIE	PIE	TMR0IE	INTE	RBIE	TMR0IF	INTF	RBIF

GIE: Global Interrupt Enable bit
1-Enables all unmasked interrupts
0-Disables all interrupts

PIE: Peripheral Interrupt Enable bit
1-Enables all unmasked peripheral interrupts
0-Disables all peripheral interrupts

TMR0IE: TMR0 Overflow Interrupt Enable bit
1-Enables the TMR0 interrupt
0-Disables the TMR0 interrupt

INTE: NA for Timers

RBIE: NA for Timers

TMR0IF: TMR0 Overflow Interrupt Flag bit
1-TMR0 register has overflowed (must be cleared in software)
0-TMR0 register did not overflow

INTF: NA for Timers

RBIF: NA for Timers

 

○ 샘플 코드

: 1ms 딜레이를 사용한 LED 블링킹 코드 샘플

  클럭:20Mhz, 인터럽트 주기 1ms, 분주비(Prescalar) : 32

  RegValue = 256-(Delay * Fosc)/(Prescalar*4)) = 256-((1ms * 20Mhz)/(32*4)) = 256-156=100

#include<pic16f877a.h>

char value = 0;
#define SBIT_PS2  2

void interrupt timer_isr()
{  
    if(TMR0IF==1)
    {
        value=~value;   // blinking the LEDs
        TMR0 = 101;     // Load the timer Value
        TMR0IF=0;       // Clear timer interrupt flag
    } 
}


void main()
{    
    TRISD=0x00;    //COnfigure PORTD as output to blink the LEDs

    OPTION_REG = (1<<SBIT_PS2);  // Timer0 with external freq and 32 as prescalar
    TMR0=100;       // Load the time value for 1ms delay
    TMR0IE=1;       //Enable timer interrupt bit in PIE1 register
    GIE=1;          //Enable Global Interrupt
    PEIE=1;         //Enable the Peripheral Interrupt

    while(1)
    {
        PORTD = value;
    }
}