
113
Atmel ATmega16/32/64/M1/C1 [DATASHEET]
7647K–AVR–12/13
13.10.3 Timer/Counter1 Control Register C – TCCR1C
Bit 7 – FOCnA: Force Output Compare for Channel A
Bit 6 – FOCnB: Force Output Compare for Channel B
The FOCnA/FOCnB bits are only active when the WGMn3:0 bits specifies a non-PWM mode. However, for ensuring
compatibility with future devices, these bits must be set to zero when TCCRnA is written when operating in a PWM mode. When
writing a logical one to the FOCnA/FOCnB bit, an immediate compare match is forced on the waveform generation unit. The
OCnA/OCnB output is changed according to its COMnx1:0 bits setting. Note that the FOCnA/FOCnB bits are implemented as
strobes. Therefore it is the value present in the COMnx1:0 bits that determine the effect of the forced compare.
A FOCnA/FOCnB strobe will not generate any interrupt nor will it clear the timer in clear timer on Compare match (CTC) mode
using OCRnA as TOP.
The FOCnA/FOCnB bits are always read as zero.
13.10.4 Timer/Counter1 – TCNT1H and TCNT1L
The two Timer/Counter I/O locations (TCNTnH and TCNTnL, combined TCNTn) give direct access, both for read and for write
operations, to the Timer/Counter unit 16-bit counter. To ensure that both the high and low bytes are read and written
simultaneously when the CPU accesses these registers, the access is performed using an 8-bit temporary high byte register
page 94. Modifying the counter (TCNTn) while the counter is running introduces a risk of missing a compare match between
TCNTn and one of the OCRnx registers.
Writing to the TCNTn register blocks (removes) the compare match on the following timer clock for all compare units.
13.10.5 Output Compare Register 1 A – OCR1AH and OCR1AL
13.10.6 Output Compare Register 1 B – OCR1BH and OCR1BL
The output compare registers contain a 16-bit value that is continuously compared with the counter value (TCNTn). A match
can be used to generate an output compare interrupt, or to generate a waveform output on the OCnx pin.
The output compare registers are 16-bit in size. To ensure that both the high and low bytes are written simultaneously when the
CPU writes to these registers, the access is performed using an 8-bit temporary high byte register (TEMP). This temporary
Bit
765
4
3
210
FOC1A
FOC1B
–
TCCR1C
Read/Write
R/W
R
Initial Value
0
Bit
7654
3210
TCNT1[15:8]
TCNT1H
TCNT1[7:0]
TCNT1L
Read/Write
R/W
Initial Value
0000
Bit
765
4321
0
OCR1A[15:8]
OCR1AH
OCR1A[7:0]
OCR1AL
Read/Write
R/W
Initial Value
000
0000
0
Bit
7654
3210
OCR1B[15:8]
OCR1BH
OCR1B[7:0]
OCR1BL
Read/Write
R/W
Initial Value
0000