Datasheet

196
ATmega640/1280/1281/2560/2561
2549K–AVR–01/07
ASSR – Asynchronous Status
Register
• 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 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 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 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 RR R R R
Initial Value 0 0 0 0 0 0 0 0