Lect05 Prog Dev

7/30/2019 Lect05 Prog Dev

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Embedded Systems

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Chapter 5

Programs Development


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Timer 0

PIC18F452 has four 16-bit timers: Timer 0, Timer 1, Timer 2, and

Timer 3. All counters are count-up type.

Timers 1-3 can be paired capture / compare / pulse-width modulation

(CCP) circuitry for external time control and measurement.

Since Timer 0 can not be paired with CCP it is usually used for

internal Looptime timing.

Timer 0 can count pulses of either the internal program clock or on pin

RA4.

Throughout the slides it is assumed that the internal clock is used.

The notation RA4 means that it is pin 4 (of pins 0-7) on I/O port A. The RA4 pin is also labeled T0CKI to show that it can be used as

Timer 0 clock inputor as a general-purpose input/output pin.

Timer 0 will be used as example in program development.


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Timer 0Features

prescaler

( N)synchronizer

TMR0H

(8-bit)

TMR0L

(8-bit)

presettable & readable

countersinternal clock

(2.5 MHz) T0CKI

(RA4)

Input

TMR0IF

(flag set on overflow)

N = 1,2,4,8, , 256

Freq < internal clock

If input = 40KHz

N = ?

If only on internal clock:

What time interval TMR0IF overflow?

How many counts need to be deducted

from the counter for overflow interval

of 10ms?


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Timer 0 The two 8-bit halves of the 16-bit Timer 0 are special function

registers (SFRs).

The most-significant byte (MSB) of the timer has symbolic name

TMR0H and is register 0xFD7.

The LSB of the timer is called TMR0L and is register 0xFD6.

All of the SFRs are listed in Appendix A8 of the book and defined in

18F452 header file.

The Timer 0 control(T0CON) - special function register sets up the

way Timer 0 functions.

Four bits of T0CON are important for internal-clock Timer 0

operation:

1) TMR0ONTimer 0 on/off2) T08BITTimer 0 8-bit/16-bit

3) T0CSTimer 0 clock select

4) PSAPrescaler use/bypass


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Timer 0 On/Off TMR0ON is a symbolic name (defined in P18F452.inc) equal to the

value 7.

The bit that turns Timer 0 on or off happens to be the most-significantbit (MSB) of T0CON (a symbolic name defined in P18F452.inc as

0xFD5).

To turn Timer 0 on, we could use:

bsf T0CON, TMR0ON ; [bsf 0xFD5,7]

Timer 0 8-bit/16-bit

Timer 0 is the only timer that can be used as an 8-bit or 16-bit timer.

Probably the only reason to use 8-bit mode is for porting code fromolder PIC C (which only had an 8-bit Timer 0).

To set Timer 0 to 16-bit mode we could use:

bcf T0CON, T08BIT


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Timer 0 Clock Select To use the RA4 pin as a clock input to Timer 0, bit T0CS of T0CON

should be 1.

To use the internal program clock as the clock input to Timer 0, bit

T0CS of T0CON should be 0.

To use the internal clock we could write:

bcf T0CON, T0CS

Timer 0 Prescaler Use/Bypass

To get a resolution of one clock cycle, bypass the prescaler.

To get a longer timer duration (at less resolution) use the prescaler.

The PSA bit of T0CON = 0 means to use prescaler ; and = 1 is to bypass

prescaler. We can write:

bsf T0CON, PSA


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Timer 0 Full Setup

To enable Timer 0 in 16-bit mode using theinternal clock and bypass the prescaler we

could write:

TMR0ON = 1, T08BIT = 0, T0CS = 0,

PSA = 1 (only underlined bits matter)

movlw B10001000

movwf T0CON


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Add a Value to a Running Timer

To shorten the cycle time of Timer 0, we need to add a

value (deducted count) to the counter by:

1) Read the 16-bit timer value in

TMR0H:TMR0L into a buffer (RAM)2) Add a 16-bit number to this value.

3) Store the result back to TMR0H:TMR0L.

The timer runs between read and store, so this

incrementing will be lost.

Note: These steps are needed because there

is no such instruction as movlf k,f

Why add?


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Instructions (TMR0LCOPY, TMR0HCOPY, andValueToAdd are defined in user code):

movff TMR0L, TMR0LCOPYmovff TMR0H, TMR0HCOPY

movlw low ValueToAdd

addwf TMR0LCOPY, F

movlw high ValueToAdd

addwfc TMR0HCOPY, F

movff TMR0HCOPY, TMR0H

movff TMR0LCOPY, TMR0L

How many cycles?


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The preceding code takes 12 clocks to execute: 2

clocks for each movff and 1 clock for other

instructions. There is also always a loss of 2 clocks on any

write to TMR0L due to synchronizer reset.

The net result is that ValueToAdd should be 14

larger than the desired value to add to Timer 0.


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Timer 0 Rollover

All PIC timers count up (increment) only andnever down (decrement).

Timer 0 sets the TMR0IF (Timer 0 interrupt flag)bit of the INTCONT (interrupt control) SFR whenthe timer counts from 0xFFFF to 0x0000(regardless of whether interrupts are used or not).

We can skip the next instruction on Timer 0rollover using:

btfss INTCON, TMR0IF


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Typical Program Structure

Ignoring interrupts, most PIC programs look like:

Initialize;

Loop forever {

Do tasks;

Wait until looptime has expired}

Refpg 66 lines 55 to 61


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Looptime Subroutine Using a 10 MHz crystal (like the QwikFlash board has) and not

using the PICs internal PLL results in an internal clock frequency

of 2.5 MHz.

Bypassing the Timer 0 prescaler for maximum timing resolution

allows for no more than 65,536 clock cycles between Timer 0

rollovers.

Using 25,000 counts at 2.5 MHz gives a nice round number of 10milliseconds per rollover.

To get 25,000 counts per rollover, we must skip the remaining

65,53625,000 = 40,536 of the maximum counts between

rollovers.

Since the code to add a value to the running counter causes 14

counts to be missed, we actually need to add 40,536 + 14 = 40,550

to the timer value.


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Symbolic Constants

The equ directive can be used to assign asymbolic name to a constant:

ValueToAdd equ 40550

If the default radix has been set to hex, the aboveresults in the symbolic name ValueToAdd beingequated with 0x9E66.

The MSB or LSB of a symbolic constant can be

obtained with the high or low directive:

high ValueToAdd ; gets 0x9E

low ValueToAdd ; gets 0x66

Bignum


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Looptime Subroutine Code

Looptime btfss INTCON, TMR0IF ;chk if Intr flag is set (10ms passed)

bra Looptime ;no! check again

movff INTCON, INTCOPY ;yes, preserve INTCON contents and

bcf INTCON, GIEH ;disable all interrupts

;reset time value

movf INTCOPY, W ;restore INTCON contents and

andlw B1000000 ;enble all interrupts

iorwf INTCON, F ;double check all interrupts are enable

bcf INTCON, TMR0IF ;clear the Intr flag

return ;returns to the main


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Macros

Macros are used to create single-linepseudo-instructions that expand tocommonly-used multi-instruction

sequences. Macro definitions have the form:

macro

endm


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Macros A macro can be definedto move a literal into a specified register

(with the side effect of trashing the W value):movlf macro literal, dest

movlw literal

movwf dest

endm

The macro MOVLF defined above can be invokedby using it like

an instruction:

movlf 17, NUM

On assembly, the macro will be expanded to act as if we had

written:

movlw 17

movwf NUM


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BlinkAlive Subroutine

It is often useful during development to have an LED that does nothing

but blink regularly when the code is running.

Toggling the LED every 10 ms would be too fast for the eye to see.

To make the LED come on every 1 second requires the BlinkAlive

subroutine to maintain a count variable that counts from 100 downto 0.

Why 100?

BlinkAlive bsf PORTA, RA4 ;off LED

decf ALIVECNT, F ;counter 1

bnz BAend ;counter != 0

movlf 100, ALIVECNT ;reset counter

bcf PORTA, RA4 ;on LED

BAend return

(Note: ALIVECNT is a user-defined variable)

invoke macro


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Using $

The $ symbol refers to the program address

of the current instruction.

An infinite loop can be generated using:

goto $

The previous instruction is equivalent to:

Loop goto Loop


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Macro Failure with Fixed Labels If we define the macro:

INFLOOP macro

FixLabelloop loop Fixlabeloopendm

The second invocation of INFLOOP will fail because of repeated fixed

label use:

INFLOOP ; OK

INFLOOP ; fails due to reuse of Fixlabel

If we define the macro:

INFLOOP macro

loop $

endm Repeated use of INFLOOP works:

INFLOOP ; OK

INFLOOP ; OK

is ok with $


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Parameters on List Directive Programs in the book start with:

list P=PIC18F452, F=INHX32, C=160, N=0, ST=OFF, MM=OFF,R=DEC, X=ON

The important parameters are:

P=PIC18F452processor is PIC18F452

F=INHX32 output is Intel HEX file

R=DEC default radix is decimal

The other parameters all have to do with what is in the list file (ref

pg66 or MPASM user manual).

Org Directive

The org assembler directive says to locate the following code in programmemory starting at the address specified:

org 0x1000 ; assemble starting at 0x1000

btg PORTC, RC2 ; the program address of this instruction is 0x1000


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Processor Include File

All of the symbolic names associated with SFRs and bits in SFRs are

defined in the processor-dependent include file:

#include P18F452.inc This include file is distributed by Microchip with its free MPLAB

development environment.

Configuration Bits The desired configuration bits can be included in the HEX file outputusing the __CONFIG command (note that it starts with a double

underscore):

__CONFIG , &

Only those bits that need to be different than the default need to be

specifed.


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Oscillator Configuration The high speed oscillator is selected using the _HS_OSC_1H bit of the

_CONFIG1H configuration word

__CONFIG _CONFIG1H, _HS_OSC_1H

This oscillator will result in a program clock that is as fast as the

crystal input.

Configure PWRT and BOR

The power-up reset timer (PWRT) and brown-out reset (BOR) could

be turned on using:

__CONFIG _CONFIG2L,_PWRT_ON_2L & _BOR_ON_2L

Dont worry about what PWRT and BOR are now.


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Automatic Register Assignment

The cblock directive automatically assigns variables sequentially to

registers starting at a give register number:

cblock 0X000 ; start at register 0x000

Variable1 ; Variable1 = 0x000

Variable2 ; Variable2 = 0x001

endc ; end of cblock

Vector Addresses

Reset vector (address executed at reset):

0x0000 High priority interrupt vector:

0x0008

Low priority interrupt vector:

0x0018


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Putting all together

All the preceding bits and pieces can be put together to

form a complete assembly language program. Refer tothe code in page 61.

Note the followings:

the general format

initialization of registers & ports

the syntax of instruction

the declaration/definition of data/variables

the macros

the directives

interrupt vectors


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What is this program doing?

movlw 6movwf COUNT

clrf SCRATCH

repeat nop

incf SCRATCH

nop

decfsz COUNT,f

goto repeat


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What is the time delay generated

if the crystal used is 10 MHz?

MOVLW 0X08MOVWF T0CON

HERE MOVLW 0X76

MOVWF TMR0H

MOVLW 0X34

MOVWF TMR0L

BCF INTC0N,TMR0IF

CALL DELAY

BRA HERE

DELAY BSF T0CON,TMR0ON

AGAIN BTFSS INTCON,TMR0IF

BRA AGAIN

BCF T0CON,TMR0ON

RETURN

;16-bit, int clock, no prescaler

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