Datasheet

Figure 20-2 Counter Unit Block Diagram
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
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).
clk
Tn
Timer/Counter clock.
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 important 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 decremented at each
timer clock (clk
Tn
). The clk
Tn
can be generated from an external or 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 clk
Tn
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 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 how waveforms are generated on the Output
Compare Outputs OCnx. For more details about advanced counting sequences and waveform
generation, refer to Modes of Operation on page 148.
The Timer/Counter Overflow (TOVn) flag is set according to the mode of operation selected by the
WGMn3:0 bits. TOVn can be used for generating a CPU interrupt.
20.6. Input Capture Unit
The Timer/Counter incorporates an Input Capture unit that can capture external events and give them a
timestamp indicating time of occurrence. The external signal indicating an event, or multiple events, can
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