Timer/Counter Clock Sources; Counter Unit - Atmel AT90PWM2 Manual

14.1.2 Registers
14.2

Timer/Counter Clock Sources

14.3

Counter Unit

4317I–AVR–01/08
The definitions in Table 14-1
Table 14-1. 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 OCR0A Register. The assignment is depen-
dent on the mode of operation.
The Timer/Counter (TCNT0) and Output Compare Registers (OCR0A and OCR0B) are 8-bit
registers. Interrupt request (abbreviated to Int.Req. in the figure) signals are all visible in the
Timer Interrupt Flag Register (TIFR0). All interrupts are individually masked with the Timer Inter-
rupt Mask Register (TIMSK0). TIFR0 and TIMSK0 are not shown in the figure.
The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source on
the T0 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
The double buffered Output Compare Registers (OCR0A and OCR0B) are 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 (OC0A and
OC0B). See "Using the Output Compare Unit" on page 113.
event will also set the Compare Flag (OCF0A or OCF0B) which can be used to generate an Out-
put Compare interrupt request.
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 (CS02:0) bits
located in the Timer/Counter Control Register (TCCR0B). For details on clock sources and pres-
caler, see "Timer/Counter0 and Timer/Counter1 Prescalers" on page
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit.
14-2 shows a block diagram of the counter and its surroundings.
Figure 14-2. Counter Unit Block Diagram
DATA BUS
TCNTn
Signal description (internal signals):
are also used extensively throughout the document.
count
clear
Control Logic
direction
bottom top
AT90PWM2/3/2B/3B
for details. The compare match
83.
TOVn
(Int.Req.)
Clock Select
Edge
Detector
clk
Tn
( From Prescaler )
).
T0
Figure
Tn
87