Rainbow Electronics ATmega128L - Manual (Page 142)

142
ATmega128(L)
2467B–09/01
clear Clear TCNT2 (set all bits to zero).
clk
Tn
Timer/counter clock, referred to as clk
T0
in the following.
top Signalize that TCNT2 has reached maximum value.
bottom Signalize that TCNT2 has reached minimum value (zero).
Depending of the mode of operation used, the counter is cleared, incremented, or dec-
remented at each timer clock (clk
T2
). clk
T2
can be generated from an external or internal
clock source, selected by the clock select bits (CS22:0). When no clock source is
selected (CS22:0 = 0) the timer is stopped. However, the TCNT2 value can be accessed
by the CPU, regardless of whether clk
T2
is present or not. A CPU write overrides (has
priority over) all counter clear or count operations.
The counting sequence is determined by the setting of the WGM01 and WGM00 bits
located in the Timer/Counter control register (TCCR2). There are close connections
between how the counter behaves (counts) and how waveforms are generated on the
output compare output OC2. For more details about advanced counting sequences and
waveform generation, see
“Modes of Operation” on page 145.
The Timer/Counter overflow (TOV2) flag is set according to the mode of operation
selected by the WGM21:0 bits. TOV2 can be used for generating a CPU interrupt.
Output Compare Unit The 8-bit comparator continuously compares TCNT2 with the output compare register
(OCR2). Whenever TCNT2 equals OCR2, the comparator signals a match. A match will
set the output compare flag (OCF2) at the next timer clock cycle. If enabled (OCIE2 = 1
and global interrupt flag in SREG is set), the output compare flag generates an output
compare interrupt. The OCF2 flag is automatically cleared when the interrupt is exe-
cuted. Alternatively, the OCF2 flag can be cleared by software by writing a logical one to
its I/O bit location. The waveform generator uses the match signal to generate an output
according to operating mode set by the WGM21:0 bits and compare output mode
(COM21:0) bits. The max and bottom signals are used by the waveform generator for
handling the special cases of the extreme values in some modes of operation (
see
“Modes of Operation” on page 145). Figure 62 shows a block diagram of the output
compare unit.