124
ATmega165A/PA/325A/PA/3250A/PA/645A/P/6450A/P [DATASHEET]
8285E–AVR–02/2013
17. Timer/Counter0 and Timer/Counter1 prescalers
Timer/Counter1 and Timer/Counter0 share the same prescaler module, but the Timer/Counters can have different
prescaler settings. The description below applies to both Timer/Counter1 and Timer/Counter0.
17.1
Prescaler Reset
The prescaler is free running, that is, operates independently of the Clock Select logic of the Timer/Counter, and it
is shared by Timer/Counter1 and Timer/Counter0. Since the prescaler is not affected by the Timer/Counter’s clock
select, the state of the prescaler will have implications for situations where a prescaled clock is used. One example
of prescaling artifacts occurs when the timer is enabled and clocked by the prescaler (6 > CSn2:0 > 1). The number
of system clock cycles from when the timer is enabled to the first count occurs can be from 1 to N+1 system clock
cycles, where N equals the prescaler divisor (8, 64, 256, or 1024).
It is possible to use the prescaler reset for synchronizing the Timer/Counter to program execution. However, care
must be taken if the other Timer/Counter that shares the same prescaler also uses prescaling. A prescaler reset
will affect the prescaler period for all Timer/Counters it is connected to.
17.2
Internal Clock Source
The Timer/Counter can be clocked directly by the system clock (by setting the CSn2:0 = 1). This provides the fast-
est operation, with a maximum Timer/Counter clock frequency equal to system clock frequency (f
CLK_I/O).
Alternatively, one of four taps from the prescaler can be used as a clock source. The prescaled clock has a fre-
quency of either f
CLK_I/O/8, fCLK_I/O/64, fCLK_I/O/256, or fCLK_I/O/1024.
17.3
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
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 17-1. 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 sam-
pling. The external clock must be guaranteed to have less than half the system 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 frequency (Nyquist sampling theorem). However, due to variation of the system
clock frequency and duty cycle caused by Oscillator source (XTAL, resonator, and capacitors) tolerances, it is rec-
ommended that maximum frequency of an external clock source is less than f
clk_I/O/2.5.
Tn_sync
(To Clock
Select Logic)
Edge Detector
Synchronization
DQ
LE
DQ
Tn
clk
I/O