ATMEGA8_14 Datasheet, PDF (77/331 Pages) ATMEL Corporation – High-performance, Low-power Atmel

ATmega8(L)
Definitions
The Input Capture Register can capture the Timer/Counter value at a given external (edge trig-
gered) event on either the Input Capture Pin (ICP1) or on the Analog Comparator pins (see
“Analog Comparator” on page 186). 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 OCR1A Register, the ICR1 Register, or by a set of fixed values. When using
OCR1A as TOP value in a PWM mode, the OCR1A 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 ICR1 Register can be used
as an alternative, freeing the OCR1A to be used as PWM output.
The following definitions are used extensively throughout the document:
Table 35. 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 OCR1A or ICR1 Register. The assignment is dependent of the mode
of operation.
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.
Accessing 16-bit
Registers
The TCNT1, OCR1A/B, and ICR1 are 16-bit registers that can be accessed by the AVR CPU via
the 8-bit data bus. The 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 16-bit read or write operation. When the Low byte of a
16-bit register is written by the CPU, the High byte stored in the temporary register, and the Low
byte written are both copied into the 16-bit register in the same clock cycle. When the Low byte
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