Файл: Microcontroller Programming. Thi Micro Chip PIC (Julio Sanchez, 2007).pdf

;|__________________________ RBPU (Pullup enable)

Once the hardware is initialized, program operation consists of reading the value stored in TMR0, scaling this value to the display range 0 to 15, and displaying it on the seven-segment LED. Processing is as follows:

;=================================

;Check value in TMR0 and display ;=================================

;Every press of the pushbutton switch connected to line

;RA4/TOCKI adds one to the value in the TMR0 register.

;Loop checks this value, adjusts to the range 0 to 15

;and displays the result in the seven-segment LED on

;Port-B

;

checkTmr0:

; Eliminate four high order bits

;At this point the w register contains a 4-bit value

;in the range 0 to 0xf. Use this value (in w) to

;obtain seven-segment display code

;

;================================

;routine to return 7-segment

;codes ;================================ segment:

The programming of seven-segment LEDs was discussed in Chapter 10.

12.3.2 Timer0 as a Simple Delay Timer

Perhaps the simplest use of the Timer0 module is to implement a delay loop. In this application, the Timer0 module is initialized to use the internal clock by setting the TOSE bit of the OPTION register. If the prescaler is to be used, as is most likely, the PSA bit is cleared and the desired prescaling is entered in bits PS2 to PS0 of the OPTION register. The circuit in Figure 12-3 allows testing several timer-related programs developed in this chapter.

The program named Timer0, in the book’s online software package, uses a timer-based delay loop to flash in sequence eight LEDs that display the binary values from 0x00 to 0xff. The delay routine executes in the foreground, so that processing is suspended while the count is in progress. The initialization requires clearing the TOCS bit in the OPTION register to select the internal clock. The prescaler is assigned to Timer0 by clearing the PSA bit and bits PS2 to PS0 are set to assign a 1:256 prescale to the timer. The following code fragment shows the processing.

6

RB0/INT RB7

7

RB1 RB6

8

RB2 RB5

9

RB3 RB4

R=330x8 Ohm

Figure 12-3 Circuit for Testing Several Timer Programs

main:

;Clear the Watchdog Timer and reset prescaler clrwdt

;Set up the OPTION register

;|__________________________ RBPU (Pullup enable)

The delay procedure named TM0delay provides the necessary time lapse between successive increments in the count displayed. The code is as follows:

;******************************

;delay sub-routine

;uses Timer0 ;****************************** TM0delay:

;Initialize the timer register

;Routine tests the value in the TMR0 register by

;subtracting 0xff from the value in TMR0. The zero flag

;is set if TMR0 = 0xff

12.3.3 Measured Time Lapse

A variable time-lapse routine that can be edited or adjusted to produce delays within a specific time range is a useful tool in any programmer’s library. In previous sections, we developed delay routines that do so by counting timer pulses. This same idea can be used to develop a routine that produces accurate delays within a range.