Timer/Counter
Clock Sources
The Timer/Counter can be clocked by an internal or an external clock source. The clock source
is selected by the clock select logic which is controlled by the clock select (CS12:0) bits located
in the Timer/Counter Control Register B (TCCR1B). For details on clock sources and prescaler,
see “Timer/Counter0 and Timer/Counter1 Prescalers” on page 74.
Counter Unit
The main part of the 16-bit Timer/Counter is the programmable 16-bit bi-directional counter unit.
Figure 33 shows a block diagram of the counter and its surroundings.
Figure 33. Counter Unit Block Diagram
DATA BUS (8-bit)
TOVn
(Int. Req.)
TEMP (8-bit)
Clock Select
count
clear
Edge
Detector
Tn
TCNTnH (8-bit) TCNTnL (8-bit)
TCNTn (16-bit Counter)
clkTn
Control Logic
direction
( From Prescaler )
TOP
BOTTOM
Signal description (internal signals):
count
Increment or decrement TCNT1 by 1.
direction Select between increment and decrement.
clear
Clear TCNT1 (set all bits to zero).
Timer/Counter clock.
clkT
1
TOP
Signalize that TCNT1 has reached maximum value.
BOTTOM Signalize that TCNT1 has reached minimum value (zero).
The 16-bit counter is mapped into two 8-bit I/O memory locations: counter high (TCNT1H) con-
taining the upper eight bits of the counter, and Counter Low (TCNT1L) containing the lower eight
bits. The TCNT1H Register can only be indirectly accessed by the CPU. When the CPU does an
access to the TCNT1H I/O location, the CPU accesses the High byte temporary register
(TEMP). The temporary register is updated with the TCNT1H value when the TCNT1L is read,
and TCNT1H is updated with the temporary register value when TCNT1L is written. This allows
the CPU to read or write the entire 16-bit counter value within one clock cycle via the 8-bit data
bus. It is important to notice that there are special cases of writing to the TCNT1 Register when
the counter is counting that will give unpredictable results. The special cases are described in
the sections where they are of importance.
Depending on the mode of operation used, the counter is cleared, incremented, or decremented
at each timer clock (clk ). The clk 1 can be generated from an external or internal clock source,
1
T
T
selected by the clock select bits (CS12:0). When no clock source is selected (CS12:0 = 0) the
timer is stopped. However, the TCNT1 value can be accessed by the CPU, independent of
whether clkT is present or not. A CPU write overrides (has priority over) all counter clear or
1
count operations.
The counting sequence is determined by the setting of the Waveform Generation mode bits
(WGM13:0) located in the Timer/Counter Control Registers A and B (TCCR1A and TCCR1B).
There are close connections between how the counter behaves (counts) and how waveforms
82
ATmega8(L)
2486T–AVR–05/08
ATMEL [ ATMEL ]
