Rainbow Electronics ATmega128L - Manual (Page 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
c lo ck s o u r ce s a n d p r e s c a l e r , se e
“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;
}