Datasheet

98
2586D–AVR–02/06
ATtiny25/45/85
Figure 16-3. Timer/Counter1 Block Diagram
Two status flags (overflow and compare match) are found in the Timer/Counter Interrupt Flag
Register - TIFR. Control signals are found in the Timer/Counter Control Registers TCCR1 and
GTCCR. The interrupt enable/disable settings are found in the Timer/Counter Interrupt Mask
Register - TIMSK.
The Timer/Counter1 contains two Output Compare Registers, OCR1A and OCR1C as the data
source to be compared with the Timer/Counter1 contents. In normal mode the Output Compare
functions are operational with OCR1A only. OCR1A determines action on the OC1A pin (PB1),
and it can generate Timer1 OC1A interrupt in normal mode and in PWM mode. OCR1C holds
the Timer/Counter maximum value, i.e. the clear on compare match value. In the normal mode
an overflow interrupt (TOV1) is generated when Timer/Counter1 counts from $FF to $00, while
in the PWM mode the overflow interrupt is generated when the Timer/Counter1 counts either
from $FF to $00 or from OCR1C to $00.
In PWM mode, OCR1A provides the data values against which the Timer Counter value is com-
pared. Upon compare match the PWM outputs (OC1A) is generated. In PWM mode, the Timer
Counter counts up to the value specified in the output compare register OCR1C and starts again
from $00. This feature allows limiting the counter “full” value to a specified value, lower than $FF.
Together with the many prescaler options, flexible PWM frequency selection is provided. Table
15-6 lists clock selection and OCR1C values to obtain PWM frequencies from 20 kHz to 250 kHz
in 10 kHz steps and from 250 kHz to 500 kHz in 50 kHz steps. Higher PWM frequencies can be
obtained at the expense of resolution.
8-BIT DATABUS
TIMER INT. FLAG
REGISTER (TIFR)
TIMER/COUNTER1
8-BIT COMPARATOR
T/C1 OUTPUT
COMPARE REGISTER
TIMER INT. MASK
REGISTER (TIMSK)
TIMER/COUNTER1
(TCNT1)
T/C CLEAR
T/C1 CONTROL
LOGIC
TOV1
TOV1
TOIE0
TOIE1
OCIE1A
OCF1A
OCF1A
CK
PCK
T/C1 OVER-
FLOW IRQ
OC1A
(PB1)
T/C1 COMPARE
MATCH A IRQ
GLOBAL T/C CONTROL
REGISTER 2 (GTCCR)
PWM1A
FOC1A
(OCR1A) (OCR1C)
8-BIT COMPARATOR
T/C1 OUTPUT
COMPARE REGISTER
TOV0
COM1A1
COM1A0
T/C CONTROL
REGISTER 1 (TCCR1)
CS12
PSR1
CS11
CS10
CS13
CTC1