ATmega640/1280/1281/2560/2561
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
(CSn2:0) bits located in the Timer/Counter control Register B (TCCRnB). For details on
clock sources and prescaler, see “Timer/Counter0, Timer/Counter1, Timer/Counter3,
Timer/Counter4, and Timer/Counter5 Prescalers” on page 169.
Counter Unit
The main part of the 16-bit Timer/Counter is the programmable 16-bit bi-directional
counter unit. Figure 50 shows a block diagram of the counter and its surroundings.
Figure 50. 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)
clkTn
Control Logic
Direction
TCNTn (16-bit Counter)
( From Prescaler )
TOP
BOTTOM
Signal description (internal signals):
Count Increment or decrement TCNTn by 1.
Direction Select between increment and decrement.
Clear
Clear TCNTn (set all bits to zero).
Timer/Counter clock.
clkT
n
TOP
Signalize that TCNTn has reached maximum value.
BOTTOM Signalize that TCNTn has reached minimum value (zero).
The 16-bit counter is mapped into two 8-bit I/O memory locations: Counter High
(TCNTnH) containing the upper eight bits of the counter, and Counter Low (TCNTnL)
containing the lower eight bits. The TCNTnH Register can only be indirectly accessed
by the CPU. When the CPU does an access to the TCNTnH I/O location, the CPU
accesses the high byte temporary register (TEMP). The temporary register is updated
with the TCNTnH value when the TCNTnL is read, and TCNTnH is updated with the
temporary register value when TCNTnL 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 impor-
tant to notice that there are special cases of writing to the TCNTn 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 dec-
remented at each timer clock (clk ). The clk can be generated from an external or
n
n
T
T
internal clock source, selected by the Clock Select bits (CSn2:0). When no clock source
is selected (CSn2:0 = 0) the timer is stopped. However, the TCNTn value can be
accessed by the CPU, independent of whether clkTn is present or not. A CPU write over-
rides (has priority over) all counter clear or count operations.
The counting sequence is determined by the setting of the Waveform Generation mode
bits (WGMn3:0) located in the Timer/Counter Control Registers A and B (TCCRnA and
TCCRnB). There are close connections between how the counter behaves (counts) and
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2549A–AVR–03/05
ATMEL [ ATMEL ]
