ATMEGA161-8PI Atmel, ATMEGA161-8PI Datasheet - Page 48

• Manufacturer Part Number: ATMEGA161-8PI
• Description: IC AVR MCU 16K 8MHZ IND 40-DIP
• Manufacturer: Atmel
• Series: AVR® ATmegar

Specifications of ATMEGA161-8PI

• Core Processor: AVR
• Core Size: 8-Bit
• Speed: 8MHz
• Connectivity: SPI, UART/USART
• Peripherals: POR, PWM, WDT
• Number Of I /o: 35
• Program Memory Size: 16KB (8K x 16)
• Program Memory Type: FLASH
• Eeprom Size: 512 x 8
• Ram Size: 1K x 8
• Voltage - Supply (vcc/vdd): 4 V ~ 5.5 V
• Oscillator Type: External
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 40-DIP (0.600", 15.24mm)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Data Converters: -
• Other names: ATMEGA1618PI

Asynchronous Operation of
Timer/Counter2
48
ATmega161(L)
When Timer/Counter2 operates asynchronously, some considerations must be taken:
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Warning: When switching between asynchronous and synchronous clocking of
Timer/Counter2, the Timer Registers TCNT2, OCR2, and TCCR2 might get
corrupted. A safe procedure for switching clock source is:
1. Disable the Timer/Counter2 interrupts by clearing OCIE2 and TOIE2.
2. Select clock source by setting AS2 as appropriate.
3. Write new values to TCNT2, OCR2, and TCCR2.
4. To switch to asynchronous operation: Wait for TCN2UB, OCR2UB, and
5. Enable interrupts, if needed.
The Oscillator is optimized for use with a 32,768 Hz watch crystal. An external clock
signal applied to this pin goes through the same amplifier having a bandwidth of
256 kHz. The external clock signal should therefore be in the interval 0 Hz -
256 kHz. The frequency of the clock signal applied to the TOSC1 pin must be lower
than one fourth of the CPU main clock frequency.
When writing to one of the registers TCNT2, OCR2, or TCCR2, the value is
transferred to a temporary register, and latched after two positive edges on TOSC1.
The user should not write a new value before the contents of the temporary register
have been transferred to its destination. Each of the three mentioned registers have
their individual temporary register, which means that e.g., writing to TCNT2 does not
disturb an OCR2 write in progress. To detect that a transfer to the destination
register has taken place, a Asynchronous Status Register – ASSR has been
implemented.
When entering Power-save mode after having written to TCNT2, OCR2 or TCCR2,
the user must wait until the written register has been updated if Timer/Counter2 is
used to wake up the device. Otherwise, the MCU will go to sleep before the changes
have had any effect. This is extremely important if the Output Compare2 interrupt is
used to wake up the device; output compare is disabled during write to OCR2 or
TCNT2. If the write cycle is not finished (i.e., the MCU enters sleep mode before the
OCR2UB bit returns to zero), the device will never get a compare match and the
MCU will not wake up.
If Timer/Counter2 is used to wake up the device from Power-save mode,
precautions must be taken if the user wants to re-enter Power-save mode: The
interrupt logic needs one TOSC1 cycle to be reset. If the time between wake-up and
re-entering Power-save mode is less than one TOSC1 cycle, the interrupt will not
occur and the device will fail to wake up. If the user is in doubt whether the time
before re-entering Power-save is sufficient, the following algorithm can be used to
ensure that one TOSC1 cycle has elapsed:
1. Write a value to TCCR2, TCNT2, or OCR2.
2. Wait until the corresponding Update Busy Flag in ASSR returns to zero.
3. Enter Power-save mode.
When asynchronous operation is selected, the 32 kHz Oscillator for Timer/Counter2
is always running, except in Power-down mode. After a Power-up Reset or wake-up
from power-down, the user should be aware of the fact that this Oscillator might take
as long as one second to stabilize. The user is advised to wait for at least one
second before using Timer/Counter2 after power-up or wake-up from power-down.
The contents of all Timer/Counter2 Registers must be considered lost after a wake-
up from power-down, due to unstable clock signal upon start-up, regardless of
whether the Oscillator is in use or a clock signal is applied to the TOSC pin.
TCR2UB.
1228D–AVR–02/07