DP83848T_07 Datasheet, PDF (33/78 Pages) National Semiconductor (TI) – Industrial Temperature Single 10/100 Ethernet Transceiver

5.2 ESD Protection
Typically, ESD precautions are predominantly in effect
when handling the devices or board before being installed
in a system. In those cases, strict handling procedures
need be implemented during the manufacturing process to
greatly reduce the occurrences of catastrophic ESD
events. After the system is assembled, internal compo-
nents are less sensitive from ESD events.
See Section 8.0 for ESD rating.
The oscillator circuit is designed to drive a parallel reso-
nance AT cut crystal with a minimum drive level of 100µW
and a maximum of 500µW. If a crystal is specified for a
lower drive level, a current limiting resistor should be
placed in series between X2 and the crystal.
As a starting point for evaluating an oscillator circuit, if the
requirements for the crystal are not known, C_L1 and C_L2
should be set at 33 pF, and R₁ should be set at 0Ω.
Specification for 25 MHz crystal are listed in Table 9 25
MHz Crystal Specification.
5.3 Clock In (X1) Recommendations
The DP83848T supports an external CMOS level oscillator
source or a crystal resonator device.
Oscillator
If an external clock source is used, X1 should be tied to the
clock source and X2 should be left floating.
The CMOS oscillator specifications for MII Mode are listed
in Table 7 25 MHz Oscillator Specification. For RMII Mode,
the CMOS oscillator specifications are listed in Table 8 50
MHz Oscillator Specification. For RMII mode, it is not rec-
ommended that the system clock out, Pin 21, be used as
the reference clock to the MAC without first verifying the
interface timing. See AN-1405 for more details.
X1
X2
R₁
C_L1
C_L2
Figure 12. Crystal Oscillator Circuit
Crystal
A 25 MHz, parallel, 20 pF load crystal resonator should be
used if a crystal source is desired. Figure 12 shows a typi-
cal connection for a crystal resonator circuit. The load
capacitor values will vary with the crystal vendors; check
with the vendor for the recommended loads.
Parameter
Frequency
Frequency
Tolerance
Frequency
Stability
Rise / Fall Time
Jitter
Jitter
Symmetry
Table 7. 25 MHz Oscillator Specification
Min
Typ
Max
25
+50
+50
40%
6
800¹
800¹
60%
Units
MHz
ppm
ppm
nsec
psec
psec
Condition
Operational Tem-
perature
1 year aging
20% - 80%
Short term
Long term
Duty Cycle
1. This limit is provided as a guideline for component selection and not guaranteed by production testing. Refer to
AN-1548, “PHYTER 100 Base-TX Reference Clock Jitter Tolerance,” for details on jitter performance.
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