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

16. A Case Study 467

LEDs and buzzer

The static output devices are tested in conjunction with the switch test listed above. Of course the failure of a LED to light or buzzer to sound may be due to either the input or output device circuit. Determining which is easily accomplished by using a voltmeter or logic probe. Also all LEDs are illuminated during the Set-time process.

Display

Each of the display devices is exercised by lighting one segment moving on once each second in an endless loop. This is implemented by generating a walking unary pattern 11111110 → 11111101 → · · · 01111111 sent out to the output subroutine SPI_WRITE once each time the file register NEW_SEC is non zero. NEW_SEC is incremented in the Timer 0 interrupt-handling routine each time the Seconds count is incremented and cleared in the Diagnostic procedure code. This acts as a ratchet giving only one new display each second.

The Set-time process is entered when the SETT switch is closed whenever the processor comes out of reset. Its function is to allow the operator to change the contents of the EEPROM Data module location 00h to any value up to 99. This location holds the initial count-down value used by the Main process to determine the length of the procedure.

The strategy behind the coding shown in Program 16.5 is to initialize the Second count to 99 and let it decrement at a 1-second rate as

Program 16.5 The Set-time process.

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

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

16. A Case Study 469

the Main process. Although this looks rather complex, it may be broken down into five phases with corresponding coding shown in Program 16.6.

Preamble

On reset if neither SETT or DIAG switches are closed the Main procedure code is entered at MAIN_PROC. This reads the initial value of the countdown period from EEPROM location 00h and initializes the count chain. The green lamp is illuminated and other lamps and buzzer are turned o .

Countdown

The Countdown phase continually displays the Minute count – updated behind the scenes by the ISR. The green lamp remains illuminated as long as this display does not drop below . This phase is complete whenever the count drops below 3 minutes or else the STOP switch is closed. In the latter case all displays are blanked and the PIC is put into its Sleep state.

In all situations except where the STOP command is issued the Minute count is displayed at 1-minute intervals. The routine at REPEAT checks the Second count and if zero the loop is repeated – that is once per minute. The simpler alternative of continually refreshing the 7-segment readouts gives an inferior display as the data being serially shifted at frequent rate may partially illuminate segments which are nominally o . In addition, repeating the loop each minute eases the task of sounding the buzzer once only when the Minute count drops to two and one.

Two minutes to go

When the display is the amber lamp is illuminated This is timed using the NEW_SEC variable. Again the loop can be prematurely exited if the STOP switch has been closed.

One minute to go

When the display is the loop diverts to illuminate the red lamp. The buzzer is sounded for two seconds; implemented in code as two 1-second buzzes.

Timed out

When the Minute count reaches zero, not only is displayed but also the buzzer sounds continually. This cacophony can only be silenced by pressing the Stop switch – or by resetting and starting again. As in previous situations when the Stop switch is closed, all displays are blanked out and the PIC is placed in its Sleep state.

Once the source code has been assembled and where possible simulated (see Fig. 8.7 on page 222) it can then be burnt into the PIC’s Program store. In the first instance only the diagnostic software and associated tasks need be programmed in order to check the target hardware. The precise details will depend somewhat on the PIC programmer being used and its associated software.

470 The Quintessential PIC Microcontroller

Program 16.6 The Main process. (continued next page).

;The 2-minutes-to-go phase ***********************************

;At a count of two sound the buzzer for one second and turn on

;the amber lamp

;The 1-minute-to-go phase ************************************

;At a count of one sound the buzzer for two second and turn on

;the red lamp

472 The Quintessential PIC Microcontroller

Fig. 16.4 Programming the PIC from MPLAB.

minute for the PIC16F84 and the 252 program words that this case study software generates.5

With F series PICs, the programming process may be repeated without any preparation of the Program store up to 100 times without any deterioration of the Flash EEPROM. C series PICS,6 such as the PIC16C74, have EPROM Program stores. Where the target has a quartz window it must be erased using a suitable UV-based EPROM eraser for approximately 20 minutes before programming, unless code is being added to previously unprogrammed memory. Although quartz windowed devices are necessary for development purposes, they are relatively expensive. Thus cheaper windowless version are used for production purposes, and are called One-Time Programmable (OTP) since they cannot subsequently be erased. Part numbers which are windowed are usually identified by JW postfix; for example, PIC16C74B-20/JW is a 20 MHz ceramic windowed PIC16C74B part and and the PIC16C74B-4/P is a 4 MHz OTP version in a 40-pin plastic DIL package. Ensure that you obtain the correct device!

The hardware and software circuits have been presented here as a simple illustrative case study to integrate many of the techniques described in the body of the text. If you decide to build your own version,files, C coding, PCB, comparison with a Motorola 68000 MPU version and other ideas

5With 772 instructions left unused, the PIC18F83 with a Program store of 512 words could be used as the target process with a small reduction in cost.

6The exception being the obsolete PIC16C83/4 which also has an EEPROM Program store.

16. A Case Study 473

Fig. 16.5 The Microchip PICSTART Plus programmer.

for experimentation, which you are welcome to contribute are given on the associated Web site detailed in the Introduction. Good luck!

474 The Quintessential PIC Microcontroller