Datasheet

19.5.3 Using the Output Compare Unit

Since writing TCNTn in any mode of operation will block all compare matches for one timer clock cycle,

there are risks involved when changing TCNTn when using the output compare unit, independently of

whether the timer/counter is running or not. If the value written to TCNTn equals the OCRnx value, the

compare match will be missed, resulting in incorrect waveform generation. Similarly, do not write the

TCNTn1 value equal to BOTTOM when the counter is counting down.

The setup of the OCnx should be performed before setting the Data Direction register for the port pin to

output. The easiest way of setting the OCnx value is to use the Force Output Compare (FOCnx) strobe

bits in Normal mode. The OCnx registers keep their values even when changing between Waveform

Generation modes.

Be aware that the TCCRnA.COMnx[1:0] bits are not double-buffered together with the compare value.

Changing the TCCRnA.COMnx[1:0] bits will take effect immediately.

19.6 Compare Match Output Unit

The Compare Output mode bits in the Timer/Counter Control Register A (TCCR0A.COM0x) have two

functions:

• The waveform generator uses the COM0x bits for defining the Output Compare (OC0x) register

state at the next compare match.

• The COM0x bits control the OC0x pin output source

The figure below shows a simplified schematic of the logic affected by COM0x. The I/O registers, I/O bits,

and I/O pins in the figure are shown in bold. Only the parts of the general I/O port control registers that

are affected by the COM0x bits are shown, namely PORT and DDR.

On system reset the OC0x register is reset to 0x00.

Note: 'OC0x state' is always referring to internal OC0x registers, not the OC0x pin.

ATmega48PA/88PA/168PA

8-bit Timer/Counter0 (TC0) with PWM

Datasheet Complete

DS40002011A-page 141