User Manual

135
ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
8271E–AVR–07/2012
If external pin modes are used for the Timer/Counter1, transitions on the T1 pin will clock the counter even if the
pin is configured as an output. This feature allows software control of the counting.
16.11.3 TCCR1C – Timer/Counter1 Control Register C
• Bit 7 – FOC1A: Force Output Compare for Channel A
• Bit 6 – FOC1B: Force Output Compare for Channel B
The FOC1A/FOC1B bits are only active when the WGM13:0 bits specifies a non-PWM mode. When writing a logi-
cal one to the FOC1A/FOC1B bit, an immediate compare match is forced on the Waveform Generation unit. The
OC1A/OC1B output is changed according to its COM1x1:0 bits setting. Note that the FOC1A/FOC1B bits are
implemented as strobes. Therefore it is the value present in the COM1x1:0 bits that determine the effect of the
forced compare.
A FOC1A/FOC1B strobe will not generate any interrupt nor will it clear the timer in Clear Timer on Compare match
(CTC) mode using OCR1A as TOP. The FOC1A/FOC1B bits are always read as zero.
16.11.4 TCNT1H and TCNT1L – Timer/Counter1
The two Timer/Counter I/O locations (TCNT1H and TCNT1L, combined TCNT1) give direct access, both for read
and for write operations, to the Timer/Counter unit 16-bit counter. To ensure that both the high and low bytes are
read and written simultaneously when the CPU accesses these registers, the access is performed using an 8-bit
temporary High Byte Register (TEMP). This temporary register is shared by all the other 16-bit registers. See
”Accessing 16-bit Registers” on page 114.
Modifying the counter (TCNT1) while the counter is running introduces a risk of missing a compare match between
TCNT1 and one of the OCR1x Registers.
Table 16-5. Clock Select Bit Description
CS12 CS11 CS10 Description
0 0 0 No clock source (Timer/Counter stopped).
001clk
I/O
/1 (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 T1 pin. Clock on falling edge.
1 1 1 External clock source on T1 pin. Clock on rising edge.
Bit 7 6 5 4 3 2 1 0
(0x82) FOC1A FOC1B – – – – – – TCCR1C
Read/Write R/W R/W R R R R R R
Initial Value 0 0 0 0 0 0 0 0
Bit 76543210
(0x85) TCNT1[15:8] TCNT1H
(0x84) TCNT1[7:0] TCNT1L
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