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 system clock frequency (fExtClk < fclk_I/O/2) given a 50/50ꢀ 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 vari-
ation 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 fclk_I/O/2.5.
An external clock source can not be prescaled.
Figure 63. Prescaler for synchronous Timer/Counters
clkI/O
Clear
PSR10
Tn
Synchronization
Tn
Synchronization
CSn0
CSn1
CSn2
CSn0
CSn1
CSn2
TIMER/COUNTERn CLOCK SOURCE
clkTn
TIMER/COUNTERn CLOCK SOURCE
clkTn
General Timer/Counter
Control Register – GTCCR
Bit
7
6
–
5
–
4
–
3
–
2
–
1
0
TSM
R/W
0
PSRASY PSRSYNC GTCCR
Read/Write
Initial Value
R
0
R
0
R
0
R
0
R
0
R/W
0
R/W
0
• Bit 7 – TSM: Timer/Counter Synchronization Mode
Writing the TSM bit to one activates the Timer/Counter Synchronization mode. In this
mode, the value that is written to the PSRASY and PSRSYNC bits is kept, hence keep-
ing the corresponding prescaler reset signals asserted. This ensures that the
corresponding Timer/Counters are halted and can be configured to the same value with-
out the risk of one of them advancing during configuration. When the TSM bit is written
to zero, the PSRASY and PSRSYNC bits are cleared by hardware, and the
Timer/Counters start counting simultaneously.
• Bit 0 – PSRSYNC: Prescaler Reset for Synchronous Timer/Counters
When this bit is one, Timer/Counter0 and Timer/Counter1, Timer/Counter3,
Timer/Counter4 and Timer/Counter5 prescaler will be Reset. This bit is normally cleared
immediately by hardware, except if the TSM bit is set. Note that Timer/Counter0,
Timer/Counter1, Timer/Counter3, Timer/Counter4 and Timer/Counter5 share the same
prescaler and a reset of this prescaler will affect all timers.
170
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATMEL [ ATMEL ]
