Interfacing PIC Microcontrollers 15 pptx

GOTO minus ; yes, minus result minus MOVLW '-' ; load minus sign BSF Select,RS ; Select data mode CALL Send ; and send symbol COMF Result ; invert all bits INCF Result ; add 1 mul MOVF N

b3 BTFSC PORTC,6 ; button 3?

RETURN

bs BTFSC PORTC,7 ; button -?

RETURN rowd BSF PORTC,2 ; select row D

BTFSC PORTC,4 ; button C?

RETURN b0 BTFSC PORTC,5 ; button 0?

RETURN

be BTFSC PORTC,6 ; button =?

RETURN

bp BTFSC PORTC,7 ; button +?

RETURN done BSF PORTC,3 ; clear last row CLRF Char ; character code = 0 RETURN

; Write display disout MOVF Kcode,W ; Load the code BSF Select,RS ; Select data mode CALL Send ; and send code RETURN

; Process operations calc MOVF Oper,W ; check for add MOVWF Temp ; load input op code MOVLW '+' ; load plus code

BTFSC STATUS,Z ; and check if same GOTO add ; yes, jump to op MOVF Oper,W ; check for subtract

BTFSC STATUS,Z

MOVF Oper,W ; check for multiply

BTFSC STATUS,Z

MOVF Oper,W ; check for divide

BTFSC STATUS,Z

GOTO scan ; rescan if key invalid

Program 6.1 Continued

; Calclate results from 2 input numbers

add MOVF Num1,W ; get first number ADDWF Num2,W ; add second MOVWF Result ; and store result

sub BSF STATUS,C ; Negative detect flag MOVF Num2,W ; get first number SUBWF Num1,W ; subtract second MOVWF Result ; and store result BTFSS STATUS,C ; answer negative?

GOTO minus ; yes, minus result

minus MOVLW '-' ; load minus sign BSF Select,RS ; Select data mode CALL Send ; and send symbol COMF Result ; invert all bits INCF Result ; add 1

mul MOVF Num1,W ; get first number CLRF Result ; total to Z add1 ADDWF Result ; add to total DECFSZ Num2 ; num2 times and GOTO add1 ; repeat if not done GOTO outres ; done, display result div CLRF Result ; total to Z MOVF Num2,W ; get divisor BCF STATUS,C ; set C flag sub1 INCF Result ; count loop start SUBWF Num1 ; subtract BTFSS STATUS,Z ; exact answer?

GOTO outres ; yes, display answer neg BTFSC STATUS,C ; gone negative?

GOTO sub1 ; no - repeat DECF Result ; correct the result MOVF Num2,W ; get divisor ADDWF Num1 ; calc remainder MOVF Result,W ; load result ADDLW 030 ; convert to ASCII BSF Select,RS ; Select data mode CALL Send ; and send result MOVLW 'r' ; indicate remainder

ADDLW 030 ; convert to ASCII

; Convert binary to BCD

outres MOVF Result,W ; load result MOVWF Lsd ; into low digit store CLRF Msd ; high digit = 0 BSF STATUS,C ; set C flag MOVLW D'10' ; load 10 again SUBWF Lsd ; sub 10 from result INCF Msd ; inc high digit BTFSC STATUS,C ; check if negative GOTO again ; no, keep going ADDWF Lsd ; yes, add 10 back DECF Msd ; inc high digit

; display 2 digit BCD result

MOVF Msd,W ; load high digit result BTFSC STATUS,Z ; check if Z

GOTO lowd ; yes, dont display Msd ADDLW 030 ; convert to ASCII BSF Select,RS ; Select data mode CALL Send ; and send Msd lowd MOVF Lsd,W ; load low digit result ADDLW 030 ; convert to ASCII BSF Select,RS ; Select data mode CALL Send ; and send Msd GOTO scan ; scan for clear key

; Restart

clear MOVLW 01 ; code to clear display BCF Select,RS ; Select data mode CALL Send ; and send code CLRF Kcount ; reset count of keys GOTO scan ; and rescan keypad END ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

Program 6.1 Continued

initialisation of the ports as required by the hardware connections is carried out A separate file for the LCD initialisation and operation has been created (LCD.INI) to keep the source code size down and to provide a re-usable file for future programs This is included at the top of the subroutine section It contains the LCD initialisation sequence (inid) and code transmission block (send), as seen in the LCD demo program in Chapter 4

The main program sequence calls the keypad scanning routine to detect a key, in which the key code is stored, and then delays for switch debouncing and release The input key is displayed, and the program then jumps to a routine to handle each input in a sequence of five buttons (Num1, Operation, Num2, Equals and Clear) The calculation routine uses the operation input code to se-lect the required process: add, subtract, multiply or divide The binary result of the calculation is passed to a routine to convert it into BCD, then ASCII, and send it to the display The result of the divide, being a single digit result and re-mainder, is sent direct to the display The clear operation sends a command to the display to clear the last set of characters

The program is highly structured to make it easier to understand (hope-fully!) In longer programs, care must be taken not to exceed the stack depth when using multiple levels of subroutine in structured programs

Pulse Output

This program illustrates the use of a hardware timer to generate an output waveform whose period can be controlled by push buttons

The hardware configuration is shown in Figure 6.3, as a screenshot The pulse output is generated at the output of Timer1, RC2, which is initialised for output

A pulse output is generated with a fixed 1 ms positive pulse, and a variable inter-val between pulses that can be adjusted manually A virtual oscilloscope displays the output waveform, which initially runs at 100 Hz The output is fed to a sounder, which causes the simulator to generate an audible output via the PC soundcard The effect of pressing each button can thus be heard as well as dis-played Note that hardware debouncing has been used (capacitors across the but-tons) to simplify the software

The main purpose of this example is to illustrate the use of Timer1 compare mode This requires pre-loading a register with a reference binary number, with which a count register is continuously compared in the timer hardware When a match is detected, an interrupt is generated which calls the interrupt service routine (ISR) This allows the input to be processed immediately, preserving the accurate timing of program operation

The Timer1 compare mode is illustrated in Figure 6.4 It uses a pair of regis-ters, TMR1H (high byte) and TMR1L (low byte), to record a 16-bit count, driven

by the MCU instruction clock In the system simulation, the clock is set to 4 MHz, giving a 1 MHz instruction clock (1 instruction takes four clock cycles) The timer therefore counts in microseconds The reference register pair, CCPR1H and CCPR1L, is pre-loaded with a value which is continuously com-pared with the 16-bit timer count (default 271016⫽ 10 00010) With this value loaded, the compare becomes true after 10 ms, the interrupt generated, and the output set high The ISR resets the interrupt, tests the buttons to see if the pre-set value should be changed, waits 1 ms and then clears the output to zero The de-fault output is therefore a 1 ms high pulse, followed by a 9 ms interval This process repeats, giving a pulse waveform with an output period of 10 ms overall The source code shows the initialisation required for the interrupt opera-tion The interrupt vector (GOTO isr) is loaded at address 004, so the initial execution sequence has to jump over this location The port, timer and inter-rupt registers are then set up (Figure 6.4 (b)) The timer is started, and the sin-gle instruction main loop then runs, waiting for the interrupt

Timer1 counts up to 10000, and the interrupt is triggered The interrupt flag is first cleared, and the counter reset to zero The next 10 ms period starts

Figure 6.3 Pulse output simulation

immediately, because the counter runs continuously The buttons are checked, and the compare register value incremented or decremented to change the output pe-riod if one of them is pressed A check is also made for zero at the upper and lower end of the period adjustment range, to prevent the compare value rolling over or under between 00 00 and FF FF This would cause the output frequency to jump between the minimum and maximum value, which is undesirable in this case The 1 ms pulse period is generated as a software delay, which runs in paral-lel with the hardware timer count After 1 ms, the output is cleared to zero, but the hardware count continues until the next interrupt occurs This is an impor-tant point – the hardware timer continues independently of the program se-quence, until the next interrupt is processed, allowing the timing operation and program to be executed simultaneously (Figure 6.5) (Program 6.2)

Period Measurement

An alternative mode of operation for Timer1 is capture mode This allows counter value in the 16-bit timer register (TMR1H ⫹ TMR1L) to be captured in mid-count; the capture is triggered by the input RC2 changing state A pre-scaler can be included between the input and capture enable so that the capture is only triggered every 4th or 16th pulse at the input, thereby reducing the capture rate

(a)

(b)

PIE1 0000 0100 Enable Timer 1 interrupt INTCON 1100 0000 Enable peripheral interrupts CCP1CON 0000 1000 Compare mode – set output pin on match CCPR1H 027H Initial value for high byte compare CCPR1L 010H Initial value for low byte compare T1CON 0000 0001 Enable timer with internal clock

CCPR1H

Comparator CCPR1L

Set Interrupt Flag (CCP1IF)

Set/Clear Pin RC2 Preload

Instruction Clock

Figure 6.4 Timer1 compare registers: (a) timer1 block diagram; (b) control registers

This method is used here to measure the period of a pulse waveform, which

is fed in at RC2, the CCP1 module input The module is set up to generate an interrupt when the input changes from high to low, once per cycle The timer counts instruction clock cycles continuously, and the count reached is stored in the CCP1 register pair on each interrupt, and the count then restarted

The main program itself continuously converts the 16-bit contents of CCP1 into 5-digit BCD, and displays the result on the LCD It could wait for the next interrupt in an idle loop, as in the pulse output program, but this program high-lights that the MCU can continue processing while the timer counts The binary

is converted into BCD using a simple subtraction loop for each digit, from ten thousands to tens (see Chapter 5) The remainder at the end is the unit’s value The simulation uses a virtual signal generator to provide a variable fre-quency square wave at RC2 The 16⫻2 LCD is connected and driven as detailed in Chapter 4 The signal generator appears full size when the simula-tion is running, as long as it is selected in the debug menu The frequency can

be adjusted as the simulation runs, and the display responds accordingly, dis-playing the period in microseconds Auto-ranging the display to milliseconds

is one possible program enhancement which could be attempted

The system operates correctly over the audio range, 15 Hz–50 kHz The ab-solute maximum count is 65536
s (16-bit count), and the minimum period is limited by the time taken to reset the interrupt and start counting again (less than 20
s) (Figures 6.6–6.8) (Program 6.3)

PULSE

Generates a variable interval pulse output controlled by up/down buttons

Hardware: P16F877 (4MHz), sounder MAIN

Initialise

RC 2/CCP1 = Pulse output

RDO,RD1 = Up/Down buttons Timer1 Compare Mode & Interrupt Wait for interrupt

SUBROUTINE 1ms delay INTERRUPT SERVICE ROUTINE Reset interrupt

IF Increase Frequency button pressed

D ecrement pulse interval

IF Decrease Frequency button pressed Increm ent pulse interval Generate 1ms pulse

Figure 6.5 Pulse program outline

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;

;

; Generates timed output interval

; using Timer 2 in compare mode

; Timer interrupt sets output, cleared after 1ms

;

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; PROCESSOR 16F877

; Clock = XT 4MHz, standard fuse settings CONFIG 0x3731

; LABEL EQUATES INCLUDE "P16F877.INC" ; Standard register labels Count EQU 20 ; soft timer

; Program begins ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ORG 0 ; Place machine code

GOTO init ; Jump over ISR vector ORG 4 ; ISR vector address GOTO isr ; run ISR

init NOP BANKSEL TRISC ; Select bank 1 MOVLW B'11111011' ; RC2 = output MOVWF TRISC ; Initialise display port MOVLW B'00000100' ; Timer1 interrupt

MOVWF PIE1 ; enable BANKSEL PORTC ; Select bank 0

MOVLW B'11000000' ; Peripheral interupt

MOVWF INTCON ; enable MOVLW B'00001000' ; Compare mode MOVWF CCP1CON ; set output on match MOVLW 027 ; Initial value

MOVWF CCPR1H ; for high byte (10ms) MOVLW 010 ; Initial value

MOVWF CCPR1L ; for low byte (10ms) MOVLW B'00000001' ; Timer1 enable

MOVWF T1CON ; internal clock (1MHz) GOTO start ; Jump to main program

; SUBROUTINES

; 1ms delay with 1us cycle time (1000 cycles) onems MOVLW D'249' ; Count for 1ms delay MOVWF Count ; Load count loop NOP ; Pad for 4 cycle loop

; INTERRUPT SERVICE ROUTINE

; Reset interrupt, check buttons, generate 1ms pulse isr CLRF PIR1 ; clear interrupt flags CLRF TMR1H ; clear timer high

CLRF TMR1L ; and low byte BTFSC PORTD,0 ; dec frequency button?

INCFSZ CCPR1H ; yes, inc period, zero?

DECF CCPR1H ; yes, step back other BTFSC PORTD,1 ; inc frequency button?

DECFSZ CCPR1H ; yes, inc period, zero?

INCF CCPR1H ; yes, step back wait CALL onems ; wait 1ms BCF CCP1CON,3 ; clear output BSF CCP1CON,3 ; re-enable timer mode

; -

; Main loop

; - start GOTO start ; wait for timer interrupt

Program 6.2 Pulse output

Figure 6.6 Input timing simulation

(a)

(b)

INTCON 1100 0000 GIE, PEIE Enable peripheral interrupts CCP1CON 0000 0100 CCP1M0 - 3 Capture mode – every falling edge T1CON 0000 0001 TMR1ON Enable timer with internal clock

Set Interrupt Flag (CCP1IF)

Pulse Input Pin RC2

Instruction Clock

Prescale &

Edge select Capture

Enable

Figure 6.7 Capture registers (a) timer registers; (b) control registers

Measure pulse waveform input period and display P16F877 (4MHz), audio signal source, !6x2 LCD MAIN

Intialise PortD = LCD outputs Capture mode & interrupt Initalise LCD

Enable capture interrupt REPEAT

Convert 16-bit count to 5 BCD digits Display input square wave period

SUBROUTINES

Convert 16-bit count to 5 BCD digits

Load 16-bit number Clear registers Tents, Thous, Hunds, Tens, Ones REPEAT

Subtract 10000 from number UNTIL Tents negative

Restore Tents and remainder REPEAT

Subtract 1000 from remainder UNTIL Thous negative

Restore Thous and remainder REPEAT

Subtract 100 from remainder UNTIL Hunds negative

Restore Hunds and remainder REPEAT

Subtract 10 from remainder UNTIL Tens negative

Restore Tens and store remainder Ones

Display input square wave period

Display ‘T=’

Supress leading zeros Display digits in ASCII Display ‘us’

INTERRUPT SERVICE ROUTINE

Clear Timer 1 Count Registers Reset interrupt flag

ALWAYS

Figure 6.8 Input period program outline

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;

;

; Measure input period using Timer1 16-bit capture

; and display in microseconds

;

; STATUS: Capture working

;

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

; Clock = XT 4MHz, standard fuse settings:

CONFIG 0x3731

; LABEL EQUATES ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

INCLUDE "P16F877.INC" ; Standard register labels

; Local label equates

Hibyte EQU 020 Lobyte EQU 021 Tents EQU 022 Thous EQU 023 Hunds EQU 024 Tens EQU 025 Ones EQU 026

; Program begins ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

GOTO ISR ; jump to service routine init NOP

BANKSEL TRISD ; Select bank 1 CLRF TRISD ; Initialise display port CLRF PIE1 ; Disable peripheral interrupts BANKSEL PORTD ; Select bank 0

CLRF PORTD ; Clear display outputs MOVLW B'11000000' ; Enable

MOVWF INTCON ; peripheral interrupts MOVLW B'00000100' ; Capture mode:

MOVWF CCP1CON ; every falling edge MOVLW B'00000001' ; Enable

MOVWF T1CON ; Timer 1 GOTO start ; Jump to main program

; INTERRUPT SERVICE ROUTINE ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

BCF PIR1,CCP1IF ; Reset interrupt flag RETFIE

Program 6.3 Input period measurement