Datasheet

8024A–AVR–04/08

ATmega8HVA/16HVA

17.5 Modes of Operation

The mode of operation is defined by the Timer/Counter Width (TCWn), Input Capture Enable

(ICENn) and the Waveform Generation Mode (WGMn0)bits in ”TCCRnA – Timer/Counter n

Control Register A” on page 90. Table 17-2 on page 80 shows the different Modes of Operation.

17.5.1 Normal 8-bit Mode

In the normal mode, the counter (TCNTnL) is incrementing until it overruns when it passes its

maximum 8-bit value (MAX = 0xFF) and then restarts from the bottom (0x00), see Table 17-2 on

page 80 for bit settings. The Overflow Flag (TOVn) will be set in the same timer clock cycle as

the TCNTnL becomes zero. The TOVn Flag in this case behaves like a ninth bit, except that it is

only set, not cleared. However, combined with the timer overflow interrupt that automatically

clears the TOVn Flag, the timer resolution can be increased by software. There are no special

cases to consider in the Normal 8-bit mode, a new counter value can be written anytime. The

Output Compare Unit can be used to generate interrupts at some given time.

17.5.2 Clear Timer on Compare Match (CTC) 8-bit Mode

In Clear Timer on Compare or CTC mode, the OCRnA Register is used to manipulate the

counter resolution, see Table 17-2 on page 80 for bit settings. In CTC mode the counter is

cleared to zero when the counter value (TCNTn) matches the OCRnA. The OCRnA defines the

top value for the counter, hence also its resolution. This mode allows greater control of the Com-

pare Match output frequency. It also simplifies the operation of counting external events.

The timing diagram for the CTC mode is shown in Figure 17-3 on page 81. The counter value

(TCNTn) increases until a Compare Match occurs between TCNTn and OCRnA, and then

counter (TCNTn) is cleared.

Table 17-2. Modes of Operation

Mode ICENn TCWn WGMn0

Timer/Counter Mode

of Operation TOP

Update of

OCRx at

TOV Flag

Set on

0 0 0 0 Normal 8-bit Mode 0xFF Immediate MAX (0xFF)

1 0 0 1 8-bit CTC OCRnA Immediate MAX (0xFF)

2 0 1 0 16-bit Mode 0xFFFF Immediate MAX (0xFFFF)

3 0 1 1 16-bit CTC

OCRnB,

OCRnA

Immediate MAX (0xFFFF)

410 0

8-bit Input Capture

mode

0xFF – MAX (0xFF)

511 0

16-bit Input Capture

mode

0xFFFF – MAX (0xFFFF)