Datasheet

118
8246A–AVR–11/09
ATtiny2313A/4313
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
< f
clk_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.
Figure 13-2. Prescaler for Timer/Counter0 and Timer/Counter1
(1)
Note: 1. The synchronization logic on the input pins (T1/T0) is shown in Figure 13-1 on page 117.
13.4 Register Description
13.4.1 GTCCR – General Timer/Counter Control Register
• Bits 7..1 – Res: Reserved Bits
These bits are reserved bits in the ATtiny2313A/4313 and will always read as zero.
• Bit 0 – PSR10: Prescaler Reset Timer/Counter1 and Timer/Counter0
When this bit is one, Timer/Counter1 and Timer/Counter0 prescaler will be Reset. This bit is nor-
mally cleared immediately by hardware. Note that Timer/Counter1 and Timer/Counter0 share
the same prescaler and a reset of this prescaler will affect both timers.
PSR10
Clear
clk
T1
clk
T0
T1
T0
clk
I/O
Synchronization
Synchronization
Bit 7 6 5 4 3 2 1 0
0x23 (0x43)
— — — — — — —PSR10GTCCR
Read/Write R R R R R R R R/W
Initial Value 0 0 0 0 0 0 0 0