116
2552K–AVR–04/11
ATmega329/3290/649/6490
16.4
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
16.5
Counter Unit
The main part of the 16-bit Timer/Counter is the programmable 16-bit bi-directional counter unit.
Figure 16-2 shows a block diagram of the counter and its surroundings.
Figure 16-2. Counter Unit Block Diagram
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).
clk
T1
Timer/Counter clock.
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
T1). The clkT1 can be generated from an external or internal clock source,
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
TEMP (8-bit)
DATA BUS (8-bit)
TCNTn (16-bit Counter)
TCNTnH (8-bit)
TCNTnL (8-bit)
Control Logic
Count
Clear
Direction
TOVn
(Int.Req.)
Clock Select
TOP
BOTTOM
Tn
Edge
Detector
( From Prescaler )
clk
Tn