Rainbow Electronics ATtiny43U - Manual (Page 80)

80
8048B–AVR–03/09
ATtiny43U
12.2.1 Registers
The Timer/Counter (TCNTn) and Output Compare Registers (OCRnA and OCRnB) are 8-bit
registers. Interrupt request (abbreviated to Int.Req. in the figure) signals are all visible in the
Timer Interrupt Flag Register (TIFRn). All interrupts are individually masked with the Timer Inter-
rupt Mask Register (TIMSKn). TIFRn and TIMSKn are not shown in the figure.
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 (clk
Tn
).
The double buffered Output Compare Registers (OCRnA and OCRnB) is compared with the
Timer/Counter value at all times. 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 (OCnA and
OCnB). See
“Output Compare Unit” on page 81 for details. The Compare Match event will also
set the Compare Flag (OCFnA or OCFnB) which can be used to generate an Output Compare
interrupt request.
12.2.2 Definitions
The definitions in Table 12-1 are used extensively throughout the document.
12.3 Timer/Counter Clock Sources
The Timer/Counter can be clocked by an internal or an external clock source. The clock source
is selected by the Clock Select logic which is controlled by the Clock Select (CSn2:0) bits
located in the Timer/Counter Control Register (TCCRnB). For details on clock sources and pres-
caler, see
“Timer/Counter Prescaler” on page 98 .
12.4 Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure
12-2 on page 81 shows a block diagram of the counter and its surroundings.
Table 12-1. Definitions
BOTTOM The counter reaches the BOTTOM when it becomes 0x00.
MAX The counter reaches its MAXimum when it becomes 0xFF (decimal 255).
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 the fixed value 0xFF
(MAX) or the value stored in the OCRnA Register. The assignment is depen-
dent on the mode of operation.