Datasheet

120
ATmega325/3250/645/6450
2570K–AVR–04/07
Figure 53. Timer/Counter Timing Diagram, with Prescaler (f
clk_I/O
/8)
16-bit Timer/Counter
Register Description
TCCR1A – Timer/Counter1
Control Register A
• Bit 7:6 – COM1A1:0: Compare Output Mode for Unit A
• Bit 5:4 – COM1B1:0: Compare Output Mode for Unit B
The COM1A1:0 and COM1B1:0 control the Output Compare pins (OC1A and OC1B
respectively) behavior. If one or both of the COM1A1:0 bits are written to one, the OC1A
output overrides the normal port functionality of the I/O pin it is connected to. If one or
both of the COM1B1:0 bit are written to one, the OC1B output overrides the normal port
functionality of the I/O pin it is connected to. However, note that the Data Direction Reg-
ister (DDR) bit corresponding to the OC1A or OC1B pin must be set in order to enable
the output driver.
When the OC1A or OC1B is connected to the pin, the function of the COM1x1:0 bits is
dependent of the WGM13:0 bits setting. Table 53 shows the COM1x1:0 bit functionality
when the WGM13:0 bits are set to a Normal or a CTC mode (non-PWM).
TOVn (FPWM)
and ICFn (if used
as TOP)
OCRnx
(Update at TOP)
TCNTn
(CTC and FPWM)
TCNTn
(PC and PFC PWM)
TOP - 1
TOP TOP - 1 TOP - 2
Old OCRnx Value New OCRnx Value
TOP - 1 TOP BOTTOM BOTTOM + 1
clk
I/O
clk
Tn
(clk
I/O
/8)
Bit 7 6 5 43210
(0x80)
COM1A1 COM1A0 COM1B1 COM1B0 – – WGM11 WGM10 TCCR1A
Read/Write R/W R/W R/W R/W R R R/W R/W
Initial Value 0 0 0 0 0 0 0 0
Table 53. Compare Output Mode, non-PWM
COM1A1/COM1B1 COM1A0/COM1B0 Description
0 0 Normal port operation, OC1A/OC1B
disconnected.
0 1 Toggle OC1A/OC1B on Compare Match.
1 0 Clear OC1A/OC1B on Compare Match (Set
output to low level).
1 1 Set OC1A/OC1B on Compare Match (Set output
to high level).