ATmega88PA Automotive Atmel Corporation, ATmega88PA Automotive Datasheet - Page 155

• Manufacturer Part Number: ATmega88PA Automotive
• Manufacturer: Atmel Corporation

18.9
9223B–AVR–09/11
Asynchronous Operation of Timer/Counter2
Figure 18-11
Figure 18-11. Timer/Counter Timing Diagram, Clear Timer on Compare Match mode, with
When Timer/Counter2 operates asynchronously, some considerations must be taken.
• Warning: When switching between asynchronous and synchronous clocking of
• The CPU main clock frequency must be more than four times the Oscillator frequency.
• When writing to one of the registers TCNT2, OCR2x, or TCCR2x, the value is transferred to
• When entering Power-save or ADC Noise Reduction mode after having written to TCNT2,
Timer/Counter2, the Timer Registers TCNT2, OCR2x, and TCCR2x might be corrupted. A
safe procedure for switching clock source is:
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 five mentioned registers have their individual temporary register,
which means that e.g. writing to TCNT2 does not disturb an OCR2x write in progress. To
detect that a transfer to the destination register has taken place, the Asynchronous Status
Register – ASSR has been implemented.
OCR2x, or TCCR2x, 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 enter sleep mode
before the changes are effective. This is particularly important if any of the Output Compare2
interrupt is used to wake up the device, since the Output Compare function is disabled during
writing to OCR2x or TCNT2. If the write cycle is not finished, and the MCU enters sleep
mode before the corresponding OCR2xUB bit returns to zero, the device will never receive a
compare match interrupt, and the MCU will not wake up.
a. Disable the Timer/Counter2 interrupts by clearing OCIE2x and TOIE2.
b. Select clock source by setting AS2 as appropriate.
c. Write new values to TCNT2, OCR2x, and TCCR2x.
d. To switch to asynchronous operation: Wait for TCN2xUB, OCR2xUB, and
e. Clear the Timer/Counter2 Interrupt Flags.
f.
Atmel ATmega48PA/88PA/168PA [Preliminary]
TCR2xUB.
Enable interrupts, if needed.
TCNTn
(clk
(CTC)
OCRnx
OCFnx
clk
clk
shows the setting of OCF2A and the clearing of TCNT2 in CTC mode.
I/O
Prescaler (f
I/O
Tn
/8)
TOP - 1
clk_I/O
/8)
TOP
TOP
BOTTOM
BOTTOM + 1
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