at90pwm81-16se ATMEL Corporation, at90pwm81-16se Datasheet - Page 91

• Manufacturer Part Number: at90pwm81-16se
• Description: 8-bit Avr Microcontroller With 8k Bytes In- System Programmable Flash
• Manufacturer: ATMEL Corporation

12.5
7734M–AVR–03/10
Input Capture Unit
Signal description (internal signals):
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 Register can only be indirectly accessed by the CPU. When the CPU does an access to the
TCNT1H I/O location, the CPU accesses the high byte temporary register (TEMP). The temporary regis-
ter 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
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
write overrides (has priority over) all counter clear or count operations.
The counting sequence is determined by the setting of the Waveform Generation mode bit (WGM13)
located in the Timer/Counter Control Registers B ( TCCR1B).
The Timer/Counter Overflow Flag (TOV1) is set according to the mode of operation selected by the
WGM13 bit. TOV1 can be used for generating a CPU interrupt.
The Timer/Counter incorporates an Input Capture unit that can capture external events and give them a
time-stamp 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 shown in
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.
Count
Clear
clk
TOP
BOTTOM
T
1
T
1
). The clk
T
1
can be generated from an external or internal clock source, selected by the
Increment TCNT1 by 1.
Clear TCNT1 (set all bits to zero).
Timer/Counter clock.
Signalize that TCNT1 has reached maximum value.
Signalize that TCNT1 has reached minimum value (zero).
Figure
12-4. The elements of the block
T
1
AT90PWM81
is present or not. A CPU
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