ATmega325PA Atmel Corporation, ATmega325PA Datasheet - Page 158

• Manufacturer Part Number: ATmega325PA
• Manufacturer: Atmel Corporation

18.11.7
8285D–AVR–06/11
GTCCR – General Timer/Counter Control Register
ATmega165A/PA/325A/PA/3250A/PA/645A/P/6450A/P
• Bit 3 – AS2: Asynchronous Timer/Counter2
When AS2 is written to zero, Timer/Counter2 is clocked from the I/O clock, clk
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, and
TCCR2A might be corrupted.
• Bit 2 – 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 1 – OCR2UB: 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 0 – TCR2UB: 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.
If a write is performed to any of the three Timer/Counter2 Registers while its update busy flag is
set, the updated value might get corrupted and cause an unintentional interrupt to occur.
The mechanisms for reading TCNT2, OCR2A, and TCCR2A are different. When reading
TCNT2, the actual timer value is read. When reading OCR2A or TCCR2A, the value in the tem-
porary storage register is read.
• Bit 1 – PSR2: Prescaler Reset Timer/Counter2
When this bit is one, the Timer/Counter2 prescaler will be reset. This bit is normally cleared
immediately by hardware. If the bit is written when Timer/Counter2 is operating in asynchronous
mode, the bit will remain one until the prescaler has been reset. The bit will not be cleared by
hardware if the TSM bit is set. Refer to the description of the
chronization Mode” on page 139
Bit
0x23 (0x43)
Read/Write
Initial Value
TSM
R/W
7
0
R
6
–
0
for a description of the Timer/Counter Synchronization mode.
R
5
–
0
R
4
–
0
R
3
–
0
”Bit 7 – TSM: Timer/Counter Syn-
R
2
–
0
PSR2
R/W
1
0
I/O
PSR10
R/W
. When AS2 is
0
0
GTCCR
158