Datasheet

91
ATmega32A [DATASHEET]
Atmel-8155D-AVR-ATmega32A-Datasheet_02/2014
Figure 17-2. 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
T
1
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) containing the upper
eight bits of the counter, and Counter Low (TCNT1L) containing the lower 8 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
T
1
). The clk
T
1
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 whether clk
T
1
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 (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 are generated on the Output Compare outputs OC1x. For more
details about advanced counting sequences and waveform generation, see “Modes of Operation” on page 103.
The Timer/Counter Overflow (TOV1) Flag is set according to the mode of operation selected by the WGM13:0 bits.
TOV1 can be used for generating a CPU interrupt.
17.6 Input Capture Unit
The Timer/Counter incorporates an Input Capture unit that can capture external events and give them a time-
stamp indicating time of occurrence. The external signal indicating an event, or multiple events, can be applied via
the ICP1 pin or alternatively, via the Analog Comparator unit. The time-stamps can then be used to calculate fre-
quency, duty-cycle, and other features of the signal applied. Alternatively the time-stamps can be used for creating
a log of the events.
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