Datasheet

The 16-bit counter is mapped into two 8-bit I/O memory locations: counter high (TCNT1H) containing the

upper eight bits of the counter, and Counter Low (TCNT1L) containing the lower eight bits. The TCNT1H

location, the CPU accesses the High byte temporary register (TEMP). The temporary register is updated

with the TCNT1H value when the TCNT1L is read, and TCNT1H is updated with the temporary register

value when TCNT1L is written. This allows the CPU to read or write the entire 16-bit counter value within

one clock cycle via the 8-bit data bus. It is important to notice that there are special cases of writing to the

TCNT1 Register when the counter is counting that will give unpredictable results. The special cases are

described in the sections where they are of importance.

Depending on the mode of operation used, the counter is cleared, incremented, or decremented at each

timer clock (clk

). The clk

can be generated from an external or internal clock source, selected by the

clock select bits (CS12:0). When no clock source is selected (CS12:0 = 0) the timer is stopped. However,

the TCNT1 value can be accessed by the CPU, independent of whether clk

is present or not. A CPU

write overrides (has priority over) all counter clear or count operations.

The counting sequence is determined by the setting of the Waveform Generation mode bits (WGM13:0)

located in the Timer/Counter Control Registers A and B (TCCR1A and TCCR1B). There are close

connections between how the counter behaves (counts) and how waveforms are generated on the

Output Compare Outputs OC1x. For more details about advanced counting sequences and waveform

generation, see Modes of Operation.

The Timer/Counter Overflow (TOV1) flag is set according to the mode of operation selected by the

WGM13:0 bits. TOV1 can be used for generating a CPU interrupt.

21.6 Input Capture Unit

The Timer/Counter incorporates an Input Capture unit that can capture external events and give them a

timestamp indicating time of occurrence. The external signal indicating an event, or multiple events, can

be applied via the ICP1 pin or alternatively, via the Analog Comparator unit. The time-stamps can then be

used to calculate frequency, duty-cycle, and other features of the signal applied. Alternatively the time-

stamps can be used for creating a log of the events.

The Input Capture unit is illustrated by the block diagram below. The elements of the block diagram that

are not directly a part of the Input Capture unit are gray shaded. The small “n” in register and bit names

indicates the Timer/Counter number.

AVR 8-Bit Microcontroller

16-bit Timer/Counter1

Datasheet Complete

40001974A-page 120