Rainbow Electronics ATtiny43U - Manual (Page 98)

98
8048B–AVR–03/09
ATtiny43U
13. Timer/Counter Prescaler
Timer/Counter0 and Timer/Counter1 share the same prescaler module, but the Timer/Counters
can have different prescaler settings. The description below applies to both 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
CLK_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 f
CLK_I/O
/8, f
CLK_I/O
/64, f
CLK_I/O
/256, or
f
CLK_I/O
/1024.
13.1 Prescaler Reset
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.
13.2 External Clock Source
An external clock source applied to the Tn pin can be used as Timer/Counter clock (clk
Tn
). The
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.
Figure 13-1 on page 98 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 (
clk
I/O
). The latch is trans-
parent in the high period of the internal system clock.
The edge detector generates one clk
T 0
pulse for each positive (CSn2:0 = 7) or negative (CSn2:0
= 6) edge it detects.
Figure 13-1. Tn 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-
Tn_sync
(To Clock
Select Logic)
Edge Detector Synchronization
D Q D Q
LE
D Q Tn
clk
I/O