Motorola MCF5281 ユーザーズマニュアル

21-10

MOTOROLA

 

move.l #0x0000,D0

move.l D0,TCN0 

;reset the counter to 0x0000

move.b #0x03,D0  ;writing ones to TER0[REF,CAP]

move.b D0,TER0 

;clears the event flags

move.w TMR0,D0

;save the contents of TMR0 while setting

bset #0,D0 

;the 0 bit. This enables timer 0 and starts counting

move.w D0,TMR0 

;load the value back into the register, setting TMR0[RST]

T0_LOOP

move.b TER0,D1 

;load TER0 and see if 

btst #1,D1 

;TER0[REF] has been set

beq T0_LOOP

addi.l #1,D2

;Increment D2

cmp.l #5,D2

;Did D2 reach 5? (i.e. timer ref has timed)

beq T0_FINISH

;If so, end timer0 example. Otherwise jump back.

move.b #0x02,D0 

;writing one to TER0[REF] clears the event flag 

move.b D0,TER0 

jmp T0_LOOP

T0_FINISH

HALT

;End processing. Example is finished

The formula below determines time-out periods for various reference values:

Time-out period = (1/clock frequency) x (1 or 16) x (DTMRn[PS] + 1) x (DTRRn[REF])

When calculating time-out periods, add 1 to the prescaler to simplify calculating, because
DTMRn[PS] = 0x00 yields a prescaler of 1 and DTMRn[PS] = 0xFF yields a prescaler of
256. For example, if a 66-MHz timer clock is divided by 16, DTMRn[PS] = 0x7F, and the
timer is referenced at 0xFBC5 (64,453 decimal), the time-out period is as follows:

Time-out period = [1/(66 x 10

6

) x (16) x (127 + 1) x (64,453) = 2.00 s