Datasheet

97
7701F–AVR–10/12
Atmel ATtiny24/44/84 [Preliminary]
14.5 Counter Unit
The main part of the 16-bit Timer/Counter is the programmable 16-bit bi-directional counter
unit. Figure 14-2 on page 97 shows a block diagram of the counter and its surroundings.
Figure 14-2. 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 Signal that TCNT1 has reached maximum value.
BOTTOM Signal 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
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 regis-
ter (TEMP). The temporary register is updated with the TCNT1H value when 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 decre-
mented at each timer clock (clk
T1
). The clk
T1
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
independently of whether clk
T1
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 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 count
ing
sequences and waveform generation, see
“Modes of Operation” on page 103.
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