Rainbow Electronics ATmega64L - Manual (Page 111)

111
ATmega16(L)
2466B–09/01
8-bit Timer/Counter2
with PWM and
Asynchronous
Operation
Timer/Counter2 is a general purpose, single channel, 8-bit Timer/Counter module. The
main features are:
•
Single Channel Counter
• Clear Timer on Compare Match (Auto Reload)
• Glitch-free, Phase Correct Pulse Width Modulator (PWM)
• Frequency Generator
• 10-bit Clock Prescaler
• Overflow and Compare Match Interrupt Sources (TOV2 and OCF2)
• Allows clocking from External 32 kHz Watch Crystal Independent of the I/O Clock
Overview A simplified block diagram of the 8-bit Timer/Counter is shown in Figure 53. For the
actual placement of I/O pins, refer to “Pinouts ATmega16” on page 2. CPU accessible
I/O registers, including I/O bits and I/O pins, are shown in bold. The device-specific I/O
register and bit locations are listed in the “8-bit Timer/Counter Register Description” on
page 121.
Figure 53. 8-bit Timer/Counter Block Diagram
Registers The Timer/Counter (TCNT2) and Output Compare Register (OCR2) are 8-bit registers.
Interrupt request (shorten as Int.Req.) signals are all visible in the Timer Interrupt Flag
Register (TIFR). All interrupts are individually masked with the Timer Interrupt Mask reg-
ister (TIMSK). TIFR and TIMSK are not shown in the figure since these registers are
shared by other timer units.
The Timer/Counter can be clocked internally, via the prescaler, or asynchronously
clocked from the TOSC1/2 pins, as detailed later in this section. The asynchronous
operation is controlled by the Asynchronous Status Register (ASSR). The Clock Select
logic block controls which clock source the Timer/Counter uses to increment (or decre-
ment) 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
T2
).
Timer/Counter
D
ATA
B
U
S
=
TCNTn
Waveform
Generation
OCn
= 0
Control Logic
= 0xFF
TOP BOTTOM
count
clear
direction
TOVn
(Int.Req.)
OCn
(Int.Req.)
Synchronization Unit
OCRn
TCCRn
ASSRn
Status flags
clk
I/O
clk
ASY
Synchronized Status flags
asynchronous mode
select (ASn)
TOSC1
T/C
Oscillator
TOSC2
Prescaler
clk
Tn
clk
I/O