Rainbow-electronics ATmega128L Uživatelský manuál stáhnout pdf (Strana 111)

111

ATmega128(L)

2467B–09/01

The following code examples show how to do an atomic write of the TCNTn register

contents. Writing any of the OCRnA/B/C or ICRn registers can be done by using the

same principle.

Note: The example code assumes that the part specific header file is included.

For I/O registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and

“SBI” instructions must be replaced with instructions that allow access to extended I/O.

Typically “LDS” and “STS” combined with “SBRS”, “SBRC”, “SBR”, and “CBR”.

The assembly code example requires that the r17:r16 register pair contains the value to

be written to TCNTn.

Reusing the Temporary High

Byte Register

If writing to more than one 16-bit register where the high byte is the same for all registers

written, then the high byte only needs to be written once. However, note that the same

rule of atomic operation described previously also applies in this case.

Timer/Counter Clock

Sources

The Timer/Counter can be clocked by an internal or an external clock source. The clock

source is selected by the clock select logic which is controlled by the clock select

(CSn2:0) bits located in the Timer/Counter control register B (TCCRnB). For details on

clock sources and prescaler, see

“Timer/Counter3, Timer/Counter2, and

Timer/Counter1 Prescalers” on page 138

Counter Unit The main part of the 16-bit Timer/Counter is the programmable 16-bit bidirectional

counter unit.

Figure 46 shows a block diagram of the counter and its surroundings.

Assembly Code Example

TIM16_WriteTCNTn:

; Save global interrupt flag

in r18,SREG

; Disable interrupts

cli

; Set TCNTn to r17:r16

out TCNTnH,r17

out TCNTnL,r16

; Restore global interrupt flag

out SREG,r18

ret

C Code Example

void TIM16_WriteTCNTn( unsigned int i )

{

unsigned char sreg;

unsigned int i;

/* Save global interrupt flag */

sreg = SREG;

/* Disable interrupts */

_CLI();

/* Set TCNTn to i */

TCNTn = i;

/* Restore global interrupt flag */

SREG = sreg;

}

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