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2010 Microchip Technology Inc.
DS39935C-page 9
ENC424J600/624J600
2.0
EXTERNAL CONNECTIONS
2.1
Oscillator
ENC424J600/624J600 devices are designed to
operate from a fixed 25 MHz clock input. This clock can
be generated by an external CMOS clock oscillator or
a parallel resonant, fundamental mode 25 MHz crystal
attached to the OSC1 and OSC2 pins. Use of a crystal,
rated for series resonant operation, will oscillate at an
incorrect frequency. To comply with IEEE 802.3 Ethernet
timing requirements, the clock must have no more than
±50 ppm of total error; avoid using resonators or clock
generators that exceed this margin.
When clocking the device using a crystal, follow the
clock oscillator or other external clock source, follow
FIGURE 2-1:
CRYSTAL OSCILLATOR
OPERATION
FIGURE 2-2:
EXTERNAL CLOCK
SOURCE
2.2
CLKOUT Pin
The Clock Out pin (CLKOUT) is provided for use as the
host controller clock or as a clock source for other
devices in the system. Its use is optional.
The 25 MHz clock applied to OSC1 is multiplied by a
PLL to internally generate a 100 MHz base clock. This
100 MHz clock is driven through a configurable
postscaler to yield a wide range of different CLKOUT
frequencies. The PLL multiplication adds clock jitter,
ensures that the clock will have a nearly ideal duty
cycle.
The CLKOUT function is enabled and the postscaler is
selected via the COCON<3:0> bits (ECON2<11:8>).
To create a clean clock signal, the CLKOUT output and
COCON bits are unaffected by all resets and
power-down modes. The CLKOUT function is enabled
out of POR and defaults to producing a 4 MHz clock.
This allows the device to directly clock the host
processor.
When the COCON bits are written with a new
configuration, the CLKOUT output transitions to the
new frequency without producing any glitches. No high
or low pulses with a shorter period than the original or
new clock are generated.
C1(3)
C2(3)
XTAL
OSC2
RS(1)
OSC1
RF(2)
To Internal Logic
Note 1: A series resistor, RS, may be required for
crystals with a low drive strength specification
or when using large loading capacitors.
2: The feedback resistor, RF , is typically 1.5 M
approx.
3: The load capacitors’ value should be derived
from the capacitive loading specification
provided by the crystal manufacture.
ENCX24J600
3.3V Clock from
External System(1)
OSC1
OSC2
Open
Note 1: Duty cycle restrictions must be observed.
ENCX24J600