ATtiny167 datasheet(112/282 Pages) ATMEL

112 / 282 page

112

7728D–AVR–07/09

ATtiny87/ATtiny167

The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source

on the Tn pin. The Clock Select logic block controls which clock source and edge the

Timer/Counter uses to increment (or decrement) its value. The Timer/Counter is inactive when

no clock source is selected. The output from the Clock Select logic is referred to as the timer

clock (clkTn).

The double buffered Output Compare Registers (OCR1A/B) are compared with the

Timer/Counter value at all time. The result of the compare can be used by the Waveform Gen-

erator to generate a PWM or variable frequency output on the Output Compare pins See

”Output Compare Units” on page 119.. The compare match event will also set the Compare

Match Flag (OCF1A/B) which can be used to generate an Output Compare interrupt request.

The Input Capture Register can capture the Timer/Counter value at a given external (edge

triggered) event on either the Input Capture pin (ICP1) or on the Analog Comparator pins (See

”AnaComp - Analog Comparator” on page 211.). The Input Capture unit includes a digital filter-

ing 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 gener-

ating 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.

12.2.2

Definitions

The following definitions are used extensively throughout the section:

12.3

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 oper-

ations. Each 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 each

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 of a 16-bit register is read by the CPU, the high byte of the 16-bit register is cop-

ied into the temporary register in the same clock cycle as the low byte is read.

Not all 16-bit accesses uses the temporary register for the high byte. Reading the OCR1A/B

16-bit registers does not involve using the temporary register.

To do a 16-bit write, the high byte must be written before the low byte. For a 16-bit read, the

low byte must be read before the high byte.

BOTTOM

The counter reaches the BOTTOM when it becomes 0x0000.

MAX

The counter reaches its MAXimum when it becomes 0xFFFF (decimal 65,535).

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 val-

ues: 0x00FF, 0x01FF, or 0x03FF, or to the value stored in the OCR1A or ICR1

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