61
ATtiny20 [DATASHEET]
8235E–AVR–03/2013
Figure 11-2. Counter Unit Block Diagram
Signal description (internal signals):
count
Increment or decrement TCNT0 by 1.
direction
Select between increment and decrement.
clear
Clear TCNT0 (set all bits to zero).
clk
T
n
Timer/Counter clock, referred to as clkT0 in the following.
top
Signalize that TCNT0 has reached maximum value.
bottom
Signalize that TCNT0 has reached minimum value (zero).
Depending of the mode of operation used, the counter is cleared, incremented, or decremented at each timer clock
(clkT0). clkT0 can be generated from an external or internal clock source, selected by the Clock Select bits (CS0[2:0]).
When no clock source is selected (CS0[2:0] = 0) the timer is stopped. However, the TCNT0 value can be accessed by
the CPU, regardless of whether clkT0 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 WGM01 and WGM00 bits located in the Timer/Counter
Control Register (TCCR0A) and the WGM02 bit located in the Timer/Counter Control Register B (TCCR0B). There are
close connections between how the counter behaves (counts) and how waveforms are generated on the Output
Compare output OC0A. For more details about advanced counting sequences and waveform generation, see
“Modes ofThe Timer/Counter Overflow Flag (TOV0) is set according to the mode of operation selected by the WGM0[1:0] bits.
TOV0 can be used for generating a CPU interrupt.
11.5
Output Compare Unit
The 8-bit comparator continuously compares TCNT0 with the Output Compare Registers (OCR0A and OCR0B).
Whenever TCNT0 equals OCR0A or OCR0B, the comparator signals a match. A match will set the Output Compare Flag
(OCF0A or OCF0B) at the next timer clock cycle. If the corresponding interrupt is enabled, the Output Compare Flag
generates an Output Compare interrupt. The Output Compare Flag is automatically cleared when the interrupt is
executed. Alternatively, the flag can be cleared by software by writing a logical one to its I/O bit location. The Waveform
Generator uses the match signal to generate an output according to operating mode set by the WGM0[2:0] bits and
Compare Output mode (COM0x[1:0]) bits. The max and bottom signals are used by the Waveform Generator for
DATA BUS
TCNTn
Control Logic
count
TOVn
(Int.Req.)
Clock Select
top
Tn
Edge
Detector
( From Prescaler )
clk
Tn
bottom
direction
clear