164
8025I–AVR–02/09
ATmega48P/88P/168P/328P
15.11.8 ASSR – Asynchronous Status Register
• Bit 7 – RES: Reserved bit
This bit is reserved and will always read as zero.
• Bit 6 – EXCLK: Enable External Clock Input
When EXCLK is written to one, and asynchronous clock is selected, the external clock input buf-
fer is enabled and an external clock can be input on Timer Oscillator 1 (TOSC1) pin instead of a
32 kHz 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 Oscil-
lator 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 hard-
ware. 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 hard-
ware. 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 hard-
ware. 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 0 – TCR2BUB: Timer/Counter Control Register2 Update Busy
When Timer/Counter2 operates asynchronously and TCCR2B is written, this bit becomes set.
When TCCR2B has been updated from the temporary storage register, this bit is cleared by
hardware. A logical zero in this bit indicates that TCCR2B is ready to be updated with a new
value.
If a write is performed to any of the five Timer/Counter2 Registers while its update busy flag is
set, the updated value might get corrupted and cause an unintentional interrupt to occur.
Bit 7 6 5 4 3 2 1 0
(0xB6) – 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
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