Datasheet

135
ATmega48/88/168 Automotive [DATASHEET]
7530K–AVR–07/14
● Reading of the TCNT2 register shortly after wake-up from power-save may give an incorrect result. Since TCNT2 is
clocked on the asynchronous TOSC clock, reading TCNT2 must be done through a register synchronized to the
internal I/O clock domain. Synchronization takes place for every rising TOSC1 edge. When waking up from
power-save mode, and the I/O clock (clk
I/O
) again becomes active, TCNT2 will read as the previous value (before
entering sleep) until the next rising TOSC1 edge. The phase of the TOSC clock after waking up from power-save
mode is essentially unpredictable, as it depends on the wake-up time. The recommended procedure for reading
TCNT2 is thus as follows:
a. Write any value to either of the registers OCR2x or TCCR2x.
b. Wait for the corresponding update busy flag to be cleared.
c. Read TCNT2.
During asynchronous operation, the synchronization of the interrupt flags for the asynchronous timer takes 3 processor
cycles plus one timer cycle. The timer is therefore advanced by at least one before the processor can read the timer value
causing the setting of the interrupt flag. The output compare pin is changed on the timer clock and is not synchronized to the
processor clock.
15.9.2 Asynchronous Status Register – ASSR
• Bit 6 – EXCLK: Enable External Clock Input
When EXCLK is written to one, and asynchronous clock is selected, the external clock input buffer is enabled and an
external clock can be input on timer oscillator 1 (TOSC1) pin instead of a 32kHz crystal. Writing to EXCLK should be done
before asynchronous operation is selected. Note that the crystal oscillator will only run when this bit is zero.
• Bit 5 – AS2: Asynchronous Timer/Counter2
When AS2 is written to zero, Timer/Counter2 is clocked from the I/O clock, clk
I/O
. When AS2 is written to one,
Timer/Counter2 is clocked from a crystal oscillator connected to the timer oscillator 1 (TOSC1) pin. When the value of AS2 is
changed, the contents of TCNT2, OCR2A, OCR2B, TCCR2A and TCCR2B might be corrupted.
• Bit 4 – TCN2UB: Timer/Counter2 Update Busy
When Timer/Counter2 operates asynchronously and TCNT2 is written, this bit becomes set. When TCNT2 has been
updated from the temporary storage register, this bit is cleared by hardware. A logical zero in this bit indicates that TCNT2 is
ready to be updated with a new value.
• Bit 3 – OCR2AUB: Output Compare Register2 Update Busy
When Timer/Counter2 operates asynchronously and OCR2A is written, this bit becomes set. When OCR2A has been
updated from the temporary storage register, this bit is cleared by hardware. A logical zero in this bit indicates that OCR2A is
ready to be updated with a new value.
• Bit 2 – OCR2BUB: Output Compare Register2 Update Busy
When Timer/Counter2 operates asynchronously and OCR2B is written, this bit becomes set. When OCR2B has been
updated from the temporary storage register, this bit is cleared by hardware. A logical zero in this bit indicates that OCR2B is
ready to be updated with a new value.
• Bit 1 – TCR2AUB: Timer/Counter Control Register2 Update Busy
When Timer/Counter2 operates asynchronously and TCCR2A is written, this bit becomes set. When TCCR2A has been
updated from the temporary storage register, this bit is cleared by hardware. A logical zero in this bit indicates that TCCR2A
is ready to be updated with a new value.
Bit 7 6 5 4 3 2 1 0
– EXCLK AS2 TCN2UB OCR2AUB OCR2BUB TCR2AUB TCR2BUB ASSR
Read/Write R R/W R/W R R R R R
Initial Value 0 0 0 0 0 0 0 0