pic assenbler tips & tricks

7/31/2019 PIC Assenbler Tips & Tricks

1/29

1999 Microchip Technology Incorporated. All Rights Reserved. S5704A Tips & Tricks 1S5704A Tips & Tricks 1 11

Tips and Tricks

Tips and TricksTips and TricksUsing PICmicroUsing PICmicro MCUsMCUs

Tips and TricksTips and TricksUsing PICmicroUsing PICmicro MCUsMCUs


2/29


Tips and Tricks

Increments and DecrementsIncrements and DecrementsIncrements and DecrementsIncrements and Decrements

q 16-Bit Incrementincfsz LOgoto $+2incf HI

q 16-Bit Decrementdecf LOincfsz LO,Wgoto $+2decf HI

q

8-Bit Decrement to 0xFFNOT_AT_FF :decf REGincfsz REG,Wgoto NOT_AT_FF

AT_FF:

q 16-Bit Incrementincfsz LOgoto $+2incf HI

q 16-Bit Decrementdecf LOincfsz LO,Wgoto $+2decf HI

q

8-Bit Decrement to 0xFFNOT_AT_FF :decf REGincfsz REG,Wgoto NOT_AT_FF

AT_FF:


3/29


Tips and Tricks

ComparisonsComparisonsComparisonsComparisons

q 8-Bit Range TestEnter with value to be tested in W.Exits with Carry set if W is in the range [LOVAL toHIVAL], inclusive.

addlw 255-HIVALaddlw (HIVAL-LOVAL)+1

q Compare and SwapCompare the values in registers X and Y. If Y < X,

swap them.

movf X,W ;GRAB X.subwf Y,W ;Y >= X?bc $+3 ;IF SO, JUMP AHEAD.addwf X ;OTHERWISE, X = X + (Y-X) = Y,

subwf Y ; AND Y = Y - (Y-X) = X.

q 8-Bit Range TestEnter with value to be tested in W.Exits with Carry set if W is in the range [LOVAL toHIVAL], inclusive.

addlw 255-HIVALaddlw (HIVAL-LOVAL)+1

q Compare and SwapCompare the values in registers X and Y. If Y < X,swap them.

movf X,W ;GRAB X.subwf Y,W ;Y >= X?bc $+3 ;IF SO, JUMP AHEAD.addwf X ;OTHERWISE, X = X + (Y-X) = Y,subwf Y ; AND Y = Y - (Y-X) = X.


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Tips and Tricks

ComparisonsComparisonsComparisonsComparisons

MinimumEnter with three values stored in registers N1, N2, and N3.Exit with min(N1,N2,N3) in MIN, N1-3 unchanged, W scrambled.

movf N1,w

subwf N2,W

movf N1,W

skpc

movf N2,W

movwf MIN

subwf N3,W

skpc

addwf MIN

MinimumEnter with three values stored in registers N1, N2, and N3.Exit with min(N1,N2,N3) in MIN, N1-3 unchanged, W scrambled.

movf N1,w

subwf N2,W

movf N1,W

skpc

movf N2,W

movwf MIN

subwf N3,W

skpc

addwf MIN


5/29


Tips and Tricks

Bit-ManipulationBit-ManipulationBit-ManipulationBit-ManipulationReverse 7 Bits

Enter with 0ABCDEFG in W. Exits with 0GFEDCBA in W.

movwf source ;source = 0ABCDEFG.

swapf source,w ;W= DEFG0ABC.

btfsc source,3 ;If D = 1, invert D and the 0.

xorlw 0x88 ;After this line, W = 0EFGDABC.

btfsc source,6 ;If A = 1, invert bits A and C.

xorlw 0x05 ;

btfsc source,4 ;If C = 1, invert A and C again.

xorlw 0x05 ;After this line, W = 0EFGDCBA.

btfsc source,2 ;Do the same with E and G.

xorlw 0x50 ;

btfsc source,0 ;

xorlw 0x50 ;After this line, W = 0GFEDCBA.

Reverse 7 BitsEnter with 0ABCDEFG in W. Exits with 0GFEDCBA in W.

movwf source ;source = 0ABCDEFG.

swapf source,w ;W= DEFG0ABC.

btfsc source,3 ;If D = 1, invert D and the 0.

xorlw 0x88 ;After this line, W = 0EFGDABC.

btfsc source,6 ;If A = 1, invert bits A and C.

xorlw 0x05 ;

btfsc source,4 ;If C = 1, invert A and C again.

xorlw 0x05 ;After this line, W = 0EFGDCBA.

btfsc source,2 ;Do the same with E and G.

xorlw 0x50 ;

btfsc source,0 ;

xorlw 0x50 ;After this line, W = 0GFEDCBA.


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Tips and Tricks

Bit-ManipulationBit-ManipulationBit-ManipulationBit-Manipulationq Rotate in Place

Rotate without inserting an extra bit from the carry.Easily extended to multi-bit rotates. Enter with ABCDEFGH inREG. Exits with BCDEFGHA in REG, W scrambled.

rlf REG,Wrlf REG

q Bit-CopyCopy bits from one register to the same position in another.

movf SOURCE,W ;The DEST bits in the positions to;which were copying must not change

xorwf DEST,W ;between this xorwf instruction and

;the xorwf DEST below.

andlw 00000111B ;A "1" in each bit-position were;copying..;this example copies the three LSBs.

xorwf DEST

q Rotate in PlaceRotate without inserting an extra bit from the carry.Easily extended to multi-bit rotates. Enter with ABCDEFGH inREG. Exits with BCDEFGHA in REG, W scrambled.

rlf REG,Wrlf REG

q Bit-CopyCopy bits from one register to the same position in another.

movf SOURCE,W ;The DEST bits in the positions to;which were copying must not change

xorwf DEST,W ;between this xorwf instruction and

;the xorwf DEST below.

andlw 00000111B ;A "1" in each bit-position were;copying..;this example copies the three LSBs.

xorwf DEST


7/29 1999 Microchip Technology Incorporated. All Rights Reserved. S5704A Tips & Tricks 1S5704A Tips & Tricks 1 77

Tips and Tricks

Bit-ManipulationBit-ManipulationBit-ManipulationBit-Manipulation

q Bit CounterCount the number of "1" bits in a register. On exit, W containsthe number of "1" bits in REG, and REG is scrambled.

rrf REG,Wandlw 0x44subwf REG

movf REG,Wandlw 0x33addwf REG

rrf REG

andlw 0x11addwf REG

rrf REGswapf REG,Waddwf REG,Wandlw 0x0F

q Bit CounterCount the number of "1" bits in a register. On exit, W containsthe number of "1" bits in REG, and REG is scrambled.

rrf REG,Wandlw 0x44

subwf REG

movf REG,Wandlw 0x33addwf REG

rrf REG

andlw 0x11addwf REG

rrf REGswapf REG,Waddwf REG,Wandlw 0x0F



Tips and Tricks

Bit-ManipulationBit-ManipulationBit-ManipulationBit-Manipulation

q Bit-Number [0-7] to Bitmask [00000001-10000000]; Enter with bit number [0-7] in BITNUM. Exits with bit

mask in W.

movlw 0x01

btfsc BITNUM,1

movlw 0x04

movwf temp

btfsc BITNUM,0

addwf temp

btfsc BITNUM,2

swapf temp

movf temp,w

q Bit-Number [0-7] to Bitmask [00000001-10000000]; Enter with bit number [0-7] in BITNUM. Exits with bit

mask in W.

movlw 0x01

btfsc BITNUM,1

movlw 0x04

movwf temp

btfsc BITNUM,0

addwf temp

btfsc BITNUM,2

swapf temp

movf temp,w



Tips and Tricks

Bit-ManipulationBit-ManipulationBit-ManipulationBit-Manipulationq

Bit-Setting without Affecting W or Statusq

Bit-Setting without Affecting W or StatusSet to zero:

clrf reg

Set to a power of two:

clrf reg

bsf reg,bit

Set to 255:

clrf reg

decfsz reg

Set a right-aligned group of bits:

clrf reg ;Set bitsbsf reg,6 ;0-5.

decfsz reg ;

Set a left-aligned group of bits:

clrf reg ;Set bits

decfsz reg ;4-7.

bcf reg,4 ;

incfsz reg ;

General method, 2-4 bits set:

clrf reg

bsf reg,bit1

bsf reg,bit2

bsf reg,bit3

bsf reg,bit4

General method, 5-7 bits set:

clrf reg

decfsz reg

bcf reg,unbit1

bcf reg,unbit2

bcf reg,unbit3



Tips and Tricks

DelaysDelaysDelaysDelaysq

Constant Delay 0-1027 Cyclesq

Constant Delay 0-1027 CyclesWAIT MACRO CYCLES

LOCAL X

IF ((CYCLES) > 1027)ERROR "MUST BE = 0 CYCLES!"ENDIF

X = (CYCLES)%4

IF (X == 1)NOPENDIF

IF (X == 2)GOTO $+1ENDIF

IF (X == 3)NOPGOTO $+1ENDIF

X = (CYCLES)/4

IF (X)

IF (X == 256)X = 0

ENDIF

MOVLW XADDLW -1SKPZGOTO $-2ENDIF

ENDM



Tips and Tricks

DelaysDelaysDelaysDelaysq

Non-Constant Delay 20-271 Cyclesq

Non-Constant Delay 20-271 Cycles SKIP:

RRF COUNTER

RRF COUNTER

MOVLW 4

SUBWF COUNTER

BCF COUNTER,6

BCF COUNTER,7

LOOP:

NOP

DECFSZ COUNTER

GOTO LOOP

RETURN

MOVLW [NUMBER OF CYCLES]

CALL DELAY

....

; DELAY 20-271 CYCLES. ENTER WITH

; NUMBER OF CYCLES TO DELAY IN W.

;

; DELAY IS INCLUSIVE OF "MOVLW",

; CALL, AND RETURN OVERHEAD.

DELAY:

MOVWF COUNTER

BTFSC COUNTER, 0

GOTO $+1

BTFSS COUNTER, 1

GOTO SKIP

NOP

GOTO $+1



Tips and Tricks

DelaysDelaysDelaysDelays

Trading Stack Space for RAMTrading Stack Space for RAM

Delay131072: call Delay16384




Delay65536: call Delay16384Delay49152: call Delay16384


















Delay192: call Delay32Delay160: call Delay32











Delay8: call Delay4

Delay4: return



Tips and Tricks

MathMathMathMath

q16-Bit Subtract with Valid Carry-Out

movf SOURCE_LO,W ;DEST = DEST - SOURCE.

subwf DEST_LO ;Note that the Z flag is

movf SOURCE_HI,W ;invalid after the

skpc ;subtraction.incfsz SOURCE_HI,W ;

subwf DEST_HI

;

q8-Bit Add with Valid Carry-In/Out

movf SOURCE,W ;DEST = DEST + SOURCE + CARRYskpnc ;Carry is valid after the

incf SOURCE,W ;addition.

addwf DEST ;



Tips and Tricks

MathMathMathMath

q RAM-Efficient Moving-Window Average

;CALCULATE NEW_AVERAGE = (255 * OLD_AVERAGE + NEW_SAMPLE)/256.

;ENTER WITH THE NEW SAMPLE IN W. EXITS WITH THE NEW AVERAGE IN W.

AVERAGE:

addwf SUMLO ;SUM = SUM + SAMPLE.movf SUMHI,W ;(AND WHILE WE'RE HERE, PUT THE OLD

skpnc ;AVERAGE IN W).

incf SUMHI ;

subwf SUMLO ;ADJUST THE SUM BY SUBTRACTING THE OLD

skpc ;AVERAGE FROM THE NEW SUM.

decf SUMHI ;

movf SUMHI,W ;W=THE NEW AVERAGE (ADJUSTED SUM/256)

return ;RETURN.



Tips and Tricks

MathMathMathMath

qTricks with Known-Zero Registers

x rlf KZ,W ;DEST = DEST + SOURCE +

CARRY.

addwf SOURCE,W ;Carry-out is only valid if SOURCE


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Tips and Tricks

MathMathMathMath

qEven Parity

;Enter with input in register "X". Exits with

; even parity in bits 1, 2, 3, 4, and 5 of

;register "X".

;; 7 words, 7 cycles.

swapf X,W

xorwf X

rrf X,Wxorwf X

rrf X,W

rlf X

xorwf X


17/29


Tips and Tricks

MathMathMathMath

qEven Parity;Enter with input in register X. Branches to "ZERO" or "ONE";depending on parity. Doesnt modify register X. For odd

;parity, replace "GOTO ZERO" with "GOTO ONE", and vice-versa.

;19 words. 5 or 7 cycles, including branch to ZERO or ONE.

SWAPF X,WXORWF X,WANDLW 00001111B

ADDWF PC

GOTO ZERO

GOTO ONEGOTO ONE

GOTO ZERO

GOTO ONE

GOTO ZERO

GOTO ZERO

GOTO ONEGOTO ONE

GOTO ZERO

GOTO ZERO

GOTO ONE

GOTO ZERO

GOTO ONEGOTO ONE

ZERO:

....

ONE:

....


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Tips and Tricks

MPASM TipsMPASM TipsMPASM TipsMPASM Tips

qEQU vs. #defineequ1 equ 5+3 ;MPASM will replace future

;occurrences of "equ1" with the

;value 8.

#define def1 5+3 ;MPASM will replace future

;occurrences of "def1" with the

;string"5+3".

x equ 3*equ1 ;This line is equivalent to "x

;equ 3*8", so x = 24.

y equ 3*def1 ;THIS line, on the other hand,

;is equivalent to "y equ 3*5+3",

;so y = 18.


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Tips and Tricks

MPASM TipsMPASM TipsMPASM TipsMPASM Tipsq

Accessing Registers Above 0x7FMethod #1:REG EQU xxx - 128 ;Generates no warnings, but also

.... ;doesnt allow REG to be viewed

MOVWF REG ;in MPLAB Watch Windows.

Method #2:MOVWF REG ;Generates an MPASM warning.

Method #3:

MOVWF REG & 0x7F ;Generates no warning, even when

;accidentally used with a

;page-0 register.Method #4:

MOVWF REG ^ 0x80 ;Generates no warning when used

;correctly, but does generate a

;warning if accidentally used

;with a page-0 register.


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Tips and Tricks

MPASM TipsMPASM TipsMPASM TipsMPASM Tips

qHIGH-and-LOW vs. Shift-and-AND

BIGNUM EQU 0x123456

LSB EQU LOW (BIGNUM) ;LSB will always be set to

;0x56.

MSB EQU HIGH (BIGNUM) ;Some versions of MPASM will

;set "MSB" to 0x12; others

;will set it to 0x34.

SAFEMSB EQU (BIGNUM>>16) & 0xFF

;This method will always set

;"SAFEMSB" to 0x12.


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Tips and Tricks

TimingTimingTimingTimingq

Exact Sync to the TMR0 Prescaler;Here, TMR0:PRE must be 01:3 or 02:0

BTFSS TMR0,BIT1GOTO $+1

; Here, TMR0:PRE must be 02:2

GOTO $+1

; TMR0 always increments from 02 to ;03 right here. When we reloadTMR0, ;we need to add 4 to the reloadvalue

;because the MOVLW/MOVWF takes 2;cycles and theres an additional 2-;cycle synchronization delay. TMR0;will resume incrementing at EXACTLY;the moment when it would have rolled;over from 03 to 04.

MOVLW RELOAD+4MOVWF TMR0

; Enter this section with TMR0; between 128 and 255, inclusive,; and the prescaler divide-by-; ratio set to divide-by-4.

WAITFOR0:

BTFSC TMR0,BIT7GOTO WAITFOR0

; Here, TMRO:PRESCALER can be; 00:2 or 00:3 or 01:0.

BTFSS TMR0,BIT0GOTO $+1

; Here, TMR0:PRE must be 01:1; or 01:2.

GOTO $+1


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Tips and Tricks

TimingTimingTimingTiming

q16-Bit Pulse-Width Measurement with 5-Cycle Resolution

;Enter with PULSE_WIDTH_HI:LO set to 0000. Exits ; ;

when PORT,PIN goes low.

CHECK_PULSE:

INCFSZ PULSE_WIDTH_LO

DECF PULSE_WIDTH_HI

BTFSC PORT,PIN

GOTO CHECK_PULSE

DONE:

MOVF PULSE_WIDTH_LO,W

ADDWF PULSE_WIDTH_HI


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Tips and Tricks

Hardware InterfacingHardware InterfacingHardware InterfacingHardware Interfacing

qZero I/O-Pin PICmicro-to- PICmicro CommunicationYour Master PICmicro controls the Slave PICmicro's MCLR line. The Master

normally keeps the Slave's MCLR line high (out of reset). When the Master

wants to send a message, it pulls the Slave's MCLR line low, then pulls it high

for a short time before pulling it low, then high again. The "message" is the time

between MCLR-low pulses.

Message error-proofing methods:

Redundancy: Send the message two or more times and require that the Slavesee consecutive, identical messages before acting on them.Parity: Send your message, then follow it with a "parity message that brings thetotal time-between-MCLR-low-pulses time to some constant value.Framing: Send a unique-length "start of message" pulse, then your messagepulse, then a unique-length "end of message pulse. The Slave doesn't acceptthe message unless it sees the "start" and "end" pulses around it.


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Tips and Tricks

qSetting ADCON0: Clock Select bits

q4 A/D clock (Tad) selections :qFosc/2, Fosc/8, Fosc/32 and Frc(internal RC)qTad Min. = 1.6 uS for VREF => 3.0V or 3.0uS for VREF < 3.0V

For fastest A/D response, calculate the Tad which is closest to the

Minimum value for Tad

Example: VREF = 5.0V, Fosc = 4.0 MHz

qFor fastest A/D response, select Fosc/8 (2.0 uS).

If Frc is selected, instead of Fosc/8, then Min. time would be 2.0us, butthe Max. time would be 6.0us I.e. 3 times slower than the best A/Dresponse time.

qFor fastest response, select Fosc = 5.0 MHz (Tad = 1.6uS)instead of 4.0Mhz (Tad = 2.0 uS).



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Tips and Tricks

q

Setting ADCON0: GO/DONEqTo start an A/D conversion the GO bit should be set to 1.Note: ADON and GO bit should not be set in the same instructionqOn conversion complete, GO/DONE = 0 & ADIF = 1.Fastest check of A/D conversion completion:

Bit test ADIF or the GO/DONE bit in tight loop .

btfsc ADCON0,DONEgoto $-1

Time for check = 3Tcycles

qChecking A/D completion using Interrupts takes:3T cycles for interrupt latency10T cycles for Context Saving + 1T cycle for ADIF check

Total = 14 T cycles



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Tips and Tricks

q

A/D Conversion in SleepqReduces digital noise during A/D conversion.

qEnable Frc mode for TadqEnable or disable A/D InterruptqStart A/D conversion and goto sleep

qOn A/D Completion:qIf interrupt is enabled then CPU wakes up from sleepqIf interrupt is disabled then CPU does not wake up from sleep,

but A/D circuit is turned OFF (ADON is still set).

qIn Crystal mode conversion in sleep is slow since most crystals take 2mS or more to wake-up

qFor fast conversion in sleep, use RC oscillator



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Tips and Tricks

MiscellaneousMiscellaneousMiscellaneousMiscellaneous

qDirect Modification of PC (or PCL)For example, to calculate 2^W,

where W is in the range [0-7]:

TBL 1,2,4,8,16,32,64,128

assembles to:

ADDWF PCL

ADDLW 0

ADDLW -1

ADDLW -3

ADDLW -7ADDLW -15

ADDLW -31

ADDLW -63

ADDLW 121

TBL: MACRO A,B,C,D,E,F,G,H

LOCAL X

ADDWF PCL

X: ADDLW HIGH (A)+1 - HIGH (B)

ADDLW HIGH (B)+1 - HIGH (C)ADDLW HIGH (C)+1 - HIGH (D)

ADDLW HIGH (D)+1 - HIGH (E)

ADDLW HIGH (E)+1 - HIGH (F)

ADDLW HIGH (F)+1 - HIGH (G)

ADDLW HIGH (G)+1 - HIGH (H)

ADDLW HIGH (H)-7

IF ((HIGH $) != (HIGH X))

ERROR "TABLE CROSSES PAGE

ERROR BOUNDARIES! DO

ERROR SOMETHING!

ENDIF

ENDM


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Tips and Tricks

MiscellaneousMiscellaneousMiscellaneousMiscellaneousq

"Switch" Statement; ENTER WITH VALUE TO BE TESTED IN W.

XORLW VAL1 ;IF W = VAL1, GOTO CASE1.

BZ CASE1 ;

XORLW VAL2^VAL1 ;IF W = VAL2, GOTO CASE2.

BZ CASE2 ;


BZ CASE3 ;


BZ CASE4 ;

etc...


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Tips and Tricks

ResourcesResourcesResourcesResources

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[email protected] the following in the message body:

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q PICLIST Archiveshttp://anick.simplenet.com/piclist/

q www.microchip.com

q Embedded Control Handbook

q PICLIST Internet Mailing ListTo subscribe, send e-mail to:

[email protected] the following in the message body:

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q Embedded Control Handbook

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