Datasheet

83
ATtiny2313/V
2543H–AVR–02/05
Registers The Timer/Counter (TCNT1), Output Compare Registers (OCR1A/B), and Input Capture
Register (ICR1) are all 16-bit registers. Special procedures must be followed when
accessing the 16-bit registers. These procedures are described in the section “Access-
ing 16-bit Registers” on page 84. The Timer/Counter Control Registers (TCCR1A/B) are
8-bit registers and have no CPU access restrictions. Interrupt requests (abbreviated to
Int.Req. in the figure) signals are all visible in the Timer Interrupt Flag Register (TIFR).
All interrupts are individually masked with the Timer Interrupt Mask Register (TIMSK).
TIFR and TIMSK are not shown in the figure.
The Timer/Counter can be clocked internally, via the prescaler, or by an external clock
source on the T1 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 (clk
T
1
).
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
Generator to generate a PWM or variable frequency output on the Output Compare pin
(OC1A/B). See “Output Compare Units” on page 91.. 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 Compar-
ator pins (See “Analog Comparator” on page 149.) 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.
Definitions The following definitions are used extensively throughout the section:
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.
Table 42. 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 Regis-
ter. The assignment is dependent of the mode of operation.