103
ATmega16M1/32M1/64M1 [DATASHEET]
8209E–AVR–11/2012
15.5
Counter unit
The main part of the 16-bit Timer/Counter is the programmable 16-bit bi-directional counter unit.
Figure 15-2 shows
a block diagram of the counter and its surroundings.
Figure 15-2. Counter unit block diagram.
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).
RTG
An external event (ICP1A or ICP1B) asks for a TOP like action.
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 clkTn 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 Generating 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
The Timer/Counter Overflow Flag (TOVn) is set according to the mode of operation selected by the WGMn3:0 bits.
TOVn can be used for generating a CPU interrupt.
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
RTG