Datasheet

The Input Capture Register can capture the Timer/Counter value at a given external (edge triggered)
event on either the Input Capture Pin (ICPn) or on the Analog Comparator pins (see Analog Comparator).
The Input Capture unit includes a digital filtering unit (Noise Canceler) for reducing the chance of
capturing noise spikes.
The TOP value, or maximum Timer/Counter value, can in some modes of operation be defined by either
the OCRnA Register, the ICRn Register, or by a set of fixed values. When using OCRnA as TOP value in
a PWM mode, the OCRnA Register can not be used for generating a PWM output. However, the TOP
value will in this case be double buffered allowing the TOP value to be changed in run time. If a fixed TOP
value is required, the ICRn Register can be used as an alternative, freeing the OCRnA to be used as
PWM output.
Related Links
AC - Analog Comparator on page 253
19.2.2. Definitions
The following definitions are used extensively throughout the document:
Table 19-1. Definitions
BOTTOM The counter reaches the BOTTOM when it becomes 0x0000.
MAX The counter reaches its MAXimum when it becomes 0xFFFF (decimal 65535).
TOP The counter reaches the TOP when it becomes equal to the highest value in the count
sequence. The TOP value can be assigned to be one of the fixed values: 0x00FF, 0x01FF, or
0x03FF, or to the value stored in the OCRnA or ICRn Register. The assignment is dependent
of the mode of operation.
19.2.3. Compatibility
The 16-bit Timer/Counter has been updated and improved from previous versions of the 16-bit AVR
Timer/Counter. This 16-bit Timer/Counter is fully compatible with the earlier version regarding:
• All 16-bit Timer/Counter related I/O Register address locations, including Timer Interrupt Registers.
• Bit locations inside all 16-bit Timer/Counter Registers, including Timer Interrupt Registers.
• Interrupt Vectors.
The following control bits have changed name, but have same functionality and register location:
• PWM10 is changed to WGM10.
• PWM11 is changed to WGM11.
• CTC1 is changed to WGM12.
The following bits are added to the 16-bit Timer/Counter Control Registers:
• FOC1A and FOC1B are added to TCCR1A.
• WGM13 is added to TCCR1B.
The 16-bit Timer/Counter has improvements that will affect the compatibility in some special cases.
19.3. Accessing 16-bit Registers
The TCNTn, OCRnA/B, and ICRn are 16-bit registers that can be accessed by the AVR CPU via the 8-bit
data bus. A 16-bit register must be byte accessed using two read or write operations. The 16-bit timer has
a single 8-bit register for temporary storing of the High byte of the 16-bit access. The same temporary
register is shared between all 16-bit registers within the 16-bit timer. Accessing the Low byte triggers the
Atmel ATmega32A [DATASHEET]
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