Файл: The quintessential PIC microcontroller (S. Katzen, 2000).pdf

16. A Case Study 461

Program 16.1 (continued.) The timebase software.

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FREEZE_EXIT

btfss PORTB,PAUSE ; Wait til switch is opened again goto FREEZE_EXIT

return

From Program 16.1 we see that time is kept as a 3-byte count chain using file registers MINUTE, SECOND and JIFFY to hold the total. Assuming that the state of bit 0 of file register Pause is 0, then one is added to the Ji y count. Normally the ISR then exits but when Ji y reaches 50 it is reset to zero and the Seconds count decremented. The file register NEW_SEC is also made non zero to indicate to background software that a second has elapsed. In the situation where the Second count reaches zero then it is reset to 59 and the Minute count decremented. The procedure is similar to the incrementing count of Example 7.4.

The Timebase task also handles the Pause function. The simplest approach would be to skip over the time decrement code if the PAUSE switch is closed. However, the necessity to keep the switch closed could be irksome if the period was more than a few minutes.

Implementing a push-on push-o scenario is ergonomically superior and can be more economically implemented in software rather than using a di erent type of switch compared to the others. In Program 16.1 the Pause handling code is located in the separate subroutine FREEZE. It is permissible to call a subroutine from an ISR in the same manner as calling one subroutine from another; that is nesting. The 8-deep hardware stack

462 The Quintessential PIC Microcontroller

allows nesting up to eight deep. In our situation only two of the stack locations are used, allowing up to six calls deeper into the stack.

Subroutine FREEZE is only entered if the PAUSE switch is closed. On each entry the value of bit 0 of the file register Pause is toggled. This is implemented by simply incrementing file register Pause.

Once Pause[0] is toggled, its state is tested and if 1 the pattern to only illuminate the two decimal points is sent to the SPI_WRITE subroutine. This is an arbitrary indicator display, another possibility would be . If Pause[0] is 0 then the state of the Minute count is sent to the OUTPUT subroutine and indicates to the user that the Pause function has ended.

Finally, the subroutine does not exit until the user releases the PAUSE switch. This is important, as on exit the ISR will be re-entered again at the next Timer 0 overflow, and this would cause Pause to be repeatedly retoggled. Some measure of switch debounce is obtained by zeroing Timer 0 and the Prescaler when the switch is released. This means that the switch will not be retested for a whole 501 second. It is for this reason that T0IF is cleared on exit from the ISR rather than at the more conventional entry point.

The task displaying the contents of the Working register in decimal is handled by the subroutine OUTPUT in Program 16.2. The task list for this function is:

1.Convert the binary datum to 2-digit BCD.

2.Convert both digits to 7-segment.

3.Serially shift out both bytes to the appropriate display.

Program 16.2 The data display function. (continued next page).

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16. A Case Study 463

Program 16.2 (continued.) The data display function.

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return SVN_SEG addwf PCL,f

retlw b’11000000’ retlw b’11111001’ retlw b’10100100’ retlw b’10110000’ retlw b’10011001’ retlw b’10010010’ retlw b’10000010’ retlw b’11111000’ retlw b’10000000’ retlw b’10010000’

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;Divide by ten

16. A Case Study 465

If the DIAG switch is closed when the PIC comes out of reset then the code transfers to the subroutine DIAGNOSTIC.

The Diagnostic process aims to exercise the various peripheral devices interfaced to the process in order to verify in a reproducible manner the status of the interconnection and the devices themselves.

466 The Quintessential PIC Microcontroller

Switches

Five switches are input via Port B. By checking each switch in turn and if closed lighting one of the LEDs or sounding the buzzer both switches and the listed output devices are tested. The DIAG switch is of course verified by moving the system into this process and the Reset switch is tested by initiating the startup process.

If there were more switches than output devices then either combinations of the latter could be activated or else one or more segments in the numerical display pushed into service.

Program 16.4 The Diagnostic process.

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;* FUNCTION: Checks each switch and activates a corresponding*

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