ATA8741 ATMEL Corporation, ATA8741 Datasheet - Page 136

• Manufacturer Part Number: ATA8741
• Manufacturer: ATMEL Corporation

22. Timer/Counter Prescaler
22.0.1
22.0.2
136
ATA8741
Prescaler Reset
External Clock Source
Timer/Counter 0, and 1 share the same prescaler module, but the Timer/Counters can have dif-
ferent prescaler settings. The description below applies to all Timer/Counters. Tn is used as a
general name, n = 0, 1.
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 (f
clock source. The prescaled clock has a frequency of either f
f
The prescaler is free running, i.e., operates independently of the Clock Select logic of the
Timer/CounterCounter, and it is shared by the Timer/Counter Tn. 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.
An external clock source applied to the Tn pin can be used as Timer/Counter clock (clk
Tn pin is sampled once every system clock cycle by the pin synchronization logic. The synchro-
nized (sampled) signal is then passed through the edge detector.
shows a functional equivalent block diagram of the Tn synchronization and edge detector logic.
The registers are clocked at the positive edge of the internal system clock (
transparent in the high period of the internal system clock.
The edge detector generates one clk
= 6) edge it detects.
Figure 22-1. 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 Tn pin to the counter is updated.
Enabling and disabling of the clock input must be done when Tn 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-
CLK_I/O
/1024.
Tn
clk
I/O
CLK_I/O
D
LE
Q
). Alternatively, one of four taps from the prescaler can be used as a
Synchronization
D
Q
T
0
pulse for each positive (CSn2:0 = 7) or negative (CSn2:0
D
CLK_I/O
Q
/8, f
Figure 22-1 on page 136
CLK_I/O
Edge Detector
/64, f
clk
I/O
9140B–INDCO–07/09
). The latch is
CLK_I/O
Tn_sync
(To Clock
Select Logic)
Tn
/256, or
). The