Datasheet
139
ATmega169A/PA/329A/PA/649A/P/3290A/PA/6490A/P [DATASHEET]
8284E–AVR–02/2013
• Bit 2:0 – CS02:0: Clock Select
The three Clock Select bits select the clock source to be used by the Timer/Counter.
If external pin modes are used for the Timer/Counter0, transitions on the T0 pin will clock the counter even if the
pin is configured as an output. This feature allows software control of the counting.
17.4.2 TCNT0 – Timer/Counter Register
The Timer/Counter Register gives direct access, both for read and write operations, to the Timer/Counter unit 8-bit
counter. Writing to the TCNT0 Register blocks (removes) the compare match on the following timer clock. Modify-
ing the counter (TCNT0) while the counter is running, introduces a risk of missing a compare match between
TCNT0 and the OCR0A Register.
17.4.3 OCR0A – Output Compare Register A
The Output Compare Register A contains an 8-bit value that is continuously compared with the counter value
(TCNT0). A match can be used to generate an Output Compare interrupt, or to generate a waveform output on the
OC0A pin.
17.4.4 TIMSK0 – Timer/Counter 0 Interrupt Mask Register
• Bit 1 – OCIE0A: Timer/Counter0 Output Compare Match A Interrupt Enable
When the OCIE0A bit is written to one, and the I-bit in the Status Register is set (one), the Timer/Counter0 Com-
pare Match A interrupt is enabled. The corresponding interrupt is executed if a compare match in Timer/Counter0
occurs, that is, when the OCF0A bit is set in the Timer/Counter 0 Interrupt Flag Register – TIFR0.
Table 17-5. Clock Select bit description.
CS02 CS01 CS00 Description
0 0 0 No clock source (Timer/Counter stopped)
001clk
I/O
/(No prescaling)
010clk
I/O
/8 (From prescaler)
011clk
I/O
/64 (From prescaler)
100clk
I/O
/256 (From prescaler)
101clk
I/O
/1024 (From prescaler)
1 1 0 External clock source on T0 pin. Clock on falling edge.
1 1 1 External clock source on T0 pin. Clock on rising edge.
Bit 76543210
0x26 (0x46) TCNT0[7:0] TCNT0
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000
Bit 76543210
0x27 (0x47) OCR0A[7:0] OCR0
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0
Bit 76543210
(0x6E)
– – – – – – OCIE0A TOIE0 TIMSK0
Read/Write RRRRRRR/WR/W
Initial Value 0 0 0 0 0 0 0 0