AT90CAN128
Timer/Counter3/1/0 Prescalers
Timer/Counter3, Timer/Counter1 and Timer/Counter0 share the same prescaler mod-
ule, but the Timer/Counters can have different prescaler settings. The description below
applies to both Timer/Counter3, Timer/Counter1 and Timer/Counter0.
Overview
Most bit references in this section are written in general form. A lower case “n” replaces
the Timer/Counter number.
Internal Clock Source
The Timer/Counter can be clocked directly by the system clock (by setting the
CSn2:0 = 1). This provides the fastest operation, with a maximum Timer/Counter clock
frequency equal to system clock frequency (fCLK_I/O). Alternatively, one of four taps from
the prescaler can be used as a clock source. The prescaled clock has a frequency of
either fCLK_I/O/8, fCLK_I/O/64, fCLK_I/O/256, or fCLK_I/O/1024.
Prescaler Reset
The prescaler is free running, i.e., operates independently of the Clock Select logic of
the Timer/Counter, and it is shared by Timer/Counter3, 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.
External Clock Source
An external clock source applied to the T3/T1/T0 pin can be used as Timer/Counter
clock (clkT3/clkT1/clkT0). The T3/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 35 shows a functional equivalent block diagram of the
T3/T1/T0 synchronization and edge detector logic. The registers are clocked at the pos-
itive edge of the internal system clock (clkI/O). The latch is transparent in the high period
of the internal system clock.
The edge detector generates one clkT3/clkT1/clkT0 pulse for each positive (CSn2:0 = 7)
or negative (CSn2:0 = 6) edge it detects.
Figure 35. T3/T1/T0 Pin Sampling
Tn_sync
(To Clock
Tn
D
Q
D
Q
D
Q
Select Logic)
LE
clkI/O
Synchronization
Edge Detector
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 T3/T1/T0 pin to the counter is
updated.
91
4250E–CAN–12/04
ATMEL [ ATMEL ]
