91
7734Q–AVR–02/12
AT90PWM81/161
11.3.1
External Clock Source
An external clock source applied to the T1/T0 pin can be used as Timer/Counter clock
(clk
T1/clkT0). The T1/T0 pin is sampled once every system clock cycle by the pin synchronization
logic. The synchronized (sampled) signal is then passed through the edge detector.
Figure 11-2shows a functional equivalent block diagram of the T1/T0 synchronization and edge detector
logic. The registers are clocked at the positive edge of the internal system clock (clk
I/O). The latch
is transparent in the high period of the internal system clock.
The edge detector generates one clk
T1/clkT0 pulse for each positive (CSn2:0 = 7) or negative
(CSn2:0 = 6) edge it detects.
Figure 11-2. T1/T0 pin sampling.
The synchronization and edge detector logic introduces a delay of 2.5 to 3.5 system clock cycles
from an edge has been applied to the T1/T0 pin to the counter is updated.
Enabling and disabling of the clock input must be done when T1/T0 has been stable for at least
one system clock cycle, otherwise it is a risk that a false Timer/Counter clock pulse is generated.
Each half period of the external clock applied must be longer than one system clock cycle to
ensure correct sampling. The external clock must be guaranteed to have less than half the sys-
tem clock frequency (f
ExtClk < fclk_I/O/2) given a 50/50% duty cycle. Since the edge detector uses
sampling, the maximum frequency of an external clock it can detect is half the sampling fre-
quency (Nyquist sampling theorem). However, due to variation of the system clock frequency
and duty cycle caused by Oscillator source (crystal, resonator, and capacitors) tolerances, it is
recommended that maximum frequency of an external clock source is less than f
clk_I/O/2.5.
An external clock source can not be prescaled.
11.4
Counter Unit
The main part of the 16-bit Timer/Counter is the programmable 16-bit bi-directional counter unit.
Figure 11-3 shows a block diagram of the counter and its surroundings.
Figure 11-3. Counter unit block diagram.
Tn_sync
(To Clock
Select Logic)
Edge Detector
Synchronization
D
Q
D
Q
LE
D
Q
Tn
clk
I/O
TEMP (8-bit)
DATA BUS
(8-bit)
TCNTn (16-bit Counter)
TCNTnH (8-bit)
TCNTnL (8-bit)
Control Logic
Count
Clear
TOVn
(Int.Req.)
Clock Select
TOP
BOTTOM
Tn
Edge
Detector
( Ckio )
clk
Tn