[elektro] mega128 clock

[Bali Zoltan]

Hali!

Nekem egy régebbi doksiban a 142. oldalon:


"External Clock Source
An external clock source applied to the Tn pin can be used as Timer/Counter clock
(clkT1/clkT2/clkT3). The Tn 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 59 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 (clkI/O). The latch is transparent in the high period of the internal
system clock.
The edge detector generates one clkT1/clkT2/clkT3 pulse for each positive (CSn2:0 = 7) or
negative (CSn2:0 = 6) edge it detects.
Figure 59. 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 system clock frequency (fExtClk < fclk_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 frequency (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 fclk_I/O/2.5."


Üdv.  Zoli

----- Original Message -----
From: "Pipi" <lista at puzsar.hu>
To: <elektro at tesla.hu>
Sent: Monday, July 21, 2008 10:53 PM
Subject: [elektro] mega128 clock


Hali!
hirtelen nem talalom az adatlapban, a  timer1-et mekkora külső frekivel lehet hajtani (előosztó nélkül)?

--
Pipi
http://www.puzsar.hu

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