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SMSC LAN91C111 32/16/8-Bit Three-In-One Fast Ethernet Controller
SMSC AN 9.6 23 Revision 1.0 (08-14-08)
APPLICATION NOTE
4 System Hardware Design
The LAN91C111 fully integrates the MAC, PHY, and SRAM into a single chip. It requires a minimum
number of external components to complete the system design. For example, it requires only a
transformer, an oscillator, some resistors, capacitors, and an optional EEPROM to complete a standard
ISA system design. For other embedded processor systems, some processors have address decode
generation logic internal to the microcontroller; the system designer should do a complete timing
analysis and add external logic between the host and the Bus Interface Unit of the LAN91C111 as
necessary. As outlined in previous sections of this manual, external logic is system dependant.
4.1 Quartz Crystal
The LAN91C111 contains on-chip oscillator circuitry. The on-chip portion is not itself an oscillator, but
an inverting amplifier. The external components that are needed to complete the oscillator circuitry are
a parallel resonant 25 MHz crystal and two capacitors (Cx1 and Cx2). When designing with a crystal,
connect the crystal to XTAL1 and XTAL2. XTAL1 is the input of the amplifier and XTAL2 is the output
of the amplifier.
Crystal Specifications
Crystal components should be mounted as close to the chip and have short, direct traces to XTAL1,
XTAL2, and VSS pins. Noise arriving at XTAL1 or XTAL2 pins can cause a miscount in the internal
clock-generating circuitry. These kinds of glitches can be produced through capacitive coupling
between the oscillator components and PCB traces carrying digital signals with fast rise and fall times.
It is also recommended not to run any high-speed signals below the area of the crystal circuitry. Hand
layout for this area of the board is recommended.
A serial resistor with value of 10W~30W may be suggested to add in serial with the XTAL2 pin of
LAN91C111 as shown in Figure 3.2, which can guarantee ( and may reduce) the maximum input
voltage not exceed the recommended level; This may also reduce the EMI affection on the Crystal
oscillator circuitry. The value of this serial resistor must be verified through Lab experiments.
A 1M Ohm resistor is recommended to be connected between XTAL1 and XTAL2 to improve the
startup link for some crystal oscillators.
25Mhz Clock Xtal1 Signal:
PARAMETER SPEC
Type Parallel Resonant
Frequency 25MHz ± 50 ppm
Duty Cycle 45% to 55%
Equivalent Series Resistance 40Ω max
Load Capacitance 20pF typical
Case Capacitance 7 pf maximum
Power Dissipation 1 mW maximum
SYMBOL PARAMETER
LIMIT
UNITMIN. TYPICAL MAX.
t1 Clock cycle period 39,998 40 40.002 ns