Rainbow Electronics ATmega128L - Manual (Page 89)

89
ATmega128(L)
2467B–09/01
Signal description (internal signals):
count Increment or decrement TCNT0 by 1.
direction Selects between increment and decrement.
clear Clear TCNT0 (set all bits to zero).
clk
T0
Timer/counter clock.
top Signalizes that TCNT0 has reached maximum value.
bottom Signalizes that TCNT0 has reached minimum value (zero).
Depending on the mode of operation used, the counter is cleared, incremented, or dec-
remented at each timer clock (clk
T0
). clk
T0
can be generated from an external or internal
clock source, selected by the clock select bits (CS02:0). When no clock source is
selected (CS02:0 = 0) the timer is stopped. However, the TCNT0 value can be accessed
by the CPU, regardless of whether clk
T0
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 (TCCR0). There are close connections
between how the counter behaves (counts) and how waveforms are generated on the
output compare output OC0. For more details about advanced counting sequences and
waveform generation, see
“Modes of Operation” on page 92.
The Timer/Counter overflow (TOV0) flag is set according to the mode of operation
selected by the WGM01:0 bits. TOV0 can be used for generating a CPU interrupt.
Output Compare Unit The 8-bit comparator continuously compares TCNT0 with the output compare register
(OCR0). Whenever TCNT0 equals OCR0, the comparator signals a match. A match will
set the output compare flag (OCF0) at the next timer clock cycle. If enabled (OCIE0 = 1),
the output compare flag generates an output compare interrupt. The OCF0 flag is auto-
matically cleared when the interrupt is executed. Alternatively, the OCF0 flag can be
cleared by software by writing a logical one to its I/O bit location. The waveform genera-
tor uses the match signal to generate an output according to operating mode set by the
WGM01:0 bits and compare output mode (COM01:0) bits. The max and bottom signals
are used by the waveform generator for handling the special cases of the extreme val-
ues in some modes of operation (
“Modes of Operation” on page 92). Figure 35 shows a
block diagram of the output compare unit.