CHAPTER 16

A Case Study

Up to this point our microcontroller material has been presented piecemeal. To complete our study we are going to put much of what we have learnt to good use and design both the hardware and software of an actual widget (gadget). This is not an easy task to do in a single short chapter. However, very little new material needs to be presented at this point, rather a process of coalescence.

We begin with our specification. Students invariably talk too long during their oral presentations. It is proposed that a dedicated embedded microcontroller-based system be designed to act as a time monitor. This monitor should default to a time-out of 10 minutes, but will have the provision to vary the allotted time from 1 to 99 minutes.

Once triggered, the monitor should perform the following sequence of operations:

1.When the GO switch is closed, a green lamp will illuminate and a

dual seven-segment display will show a count-down from the timeout value to at one-minute intervals.

2.After a further minute, an amber lamp only will illuminate, the count of will be displayed and a buzzer will sound for nominally one second.

3.After a further minute, a red lamp only will illuminate together with a display of . The buzzer will sound for two seconds.

4.Finally, after another minute the display will show , the red lamp will continue to be illuminated and the buzzer will sound continuously until the STOP switch is pressed. This will halt the timer and turn o all displays, lamps and buzzer. Indeed, closing the STOP switch at any time during the sequence above will cause the system to permanently halt. The system may be restarted from the time-out value by resetting the processor.

5.At any time the sequence can be frozen by toggling the PAUSE switch. When toggled again, the sequence will continue on from where it left o .

6.In order to alter the time-out from the default value of , the SET switch must be closed when the system is reset. The display will then show and will count down slowly. The value showing when the

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Switches

The five switches S2…S6 implementing the functions GO, SET, STOP, DIAG, PAUSE are read from Port B at RB[4:0]. By using this port’s internal pull-up resistors (see Fig 11.7 on page 280) no external resistors are required.

S1 with R1 provides a Manual reset in order to restart the count. This MCLR signal also provides a Reset signal feed for external circuitry.

All six switches can be conveniently implemented as momentary contact keyboard switches.

Lamps

Three suitably colored 10 mm (0.4") high-brightness LEDs D3…D1 driven from RB[7:5] provide the light signals. 330Ω series resistors limit the current to nominally 10 mA.

Buzzer

The buzzer should be a miniature solid-state device. A typical piezoelectric implementation will operate over a wide d.c. voltage range of typically 3–16 V and require little more than 1 mA at 5 V.1

The buzzer is driven via RA2.

Numerical display

Two 7-segment displays give the required 2-digit read-out, facilitating the maximum specified period of 99 minutes. As only four port pins remain, a serial interface is implemented. This is similar to that shown in Fig. 12.2 on page 307 but each SIPO shift register has a separate data feed, with RA0 being used for the ten’s digit and RA3 for the units digit. Both digits can therefore be simultaneously updated with eight shifts.

The common-anode seven-segment display pinning shown in the diagram is that of the 16-pin Dual In Line (DIL) footprint with both left and right decimal points – lhdp and rhdp. Only the latter is used here (to indicate that the system has paused) in conjunction with the 8-bit 74HC164 shift register. Alternative 16and 14-pinouts are commonly available and even dual-digit packages. However, even the 16-pin footprint pinout is not standardized.

Smaller-sized displays, typically below 0.8 /20 mm, use a single LED for each bar, with a conducting voltage drop of around 2 V.2 The DIL 330 Ω series resistors R5 and R6 limit the current to around 10 mA. The common anodes are connected directly back to the normal +5 V power supply to avoid current surges a ecting the logic circuits, and should be decoupled by small tantalum capacitors. Although the displays are normally rated for 20 mA, restricting the current to this value gives suf-

1If you want to put paid to any possibility of the speaker continuing, a piezo-electric sound bomb producing 110 dB at 1 m distance needs a 12 V d.c. supply at 200 mA.

2Larger displays, e.g. 2.24 /56 mm, have typically two or four LEDs in series. In the latter case a separate 12 V supply would be needed and current bu ering.

16. A Case Study 459

Timebase task

All processes are time related. Timekeeping is implemented in hardware by generating an interrupt 50 times each second. By keeping a Ji y count, seconds and minute tasks are updated and are used to sequence the appropriate process.

By monitoring the PAUSE switch this decrementing time chain can be by-passed, hence freezing the countdown for as long as necessary.

Display task

All processes need to output the state of the count or status information to the two 7-segment displays. As this involves parallel to serial conversion and shifting, the task is better gathered into one module.

Main process

The Main process is a loop displaying the Minute count until it reaches zero, with a premature break if the STOP switch is closed.

Set-time process

If the SET switch is closed when the PIC is reset then the SET_TIME subroutine quickly decrements the display count until the switch is released. This displayed value is then written into Data EEPROM and is used by all subsequent Main processes as the starting value for the Minute count.

Diagnostic process

If the DIAG switch is closed on reset, the system enters a diagnostic subroutine. The essentially exercises each peripheral device in a manner calculated to ease hardware fault finding.

All processes are dependent on the Timebase task to pass basic realtime clock information back. As shown in Program 16.1 this is interrupt driven and is based on the Timer 0:Prescaler dividing down the 3.2763 MHz crystal-driven oscillator to give overflow every 501 s. As can be seen in Program 16.3, the Timer 0 interrupt is enabled and thus the PIC will enter ISR whenever the timer overflows – every 256 outputs from the Prescaler. Remembering that the crystal oscillator runs at 14 of the crystal frequency, a prescale ratio of 1:64 will give a timebase rate of 50

per second – 3.2763×106 = 50.

4×64×256

The task list for this function is:

1.IF PAUSE switch open THEN

(a)Decrement the time chain by one Ji y.

(b)IF new second THEN flag it.

2.ELSE

(a)Toggle the Pause flag.

(b)IF set THEN tell the world that the system is paused.

(c)ELSE display time to indicate normal running.

(d)Wait until PAUSE switch is released.

3.Return from interrupt.

460 The Quintessential PIC Microcontroller

Program 16.1 The timebase software. (continued next page).

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

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

;First save context in usual way

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

;The core code

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

16. A Case Study 461

Program 16.1 (continued.) The timebase software.

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

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

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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Program 16.2 (continued.) The data display function.

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

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

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’

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

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

;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.

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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.

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

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

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

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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.

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

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

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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.

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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

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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.

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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