SMSC LAN91C111 32/16/8-Bit Three-In-One Fast Ethernet Controller

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.

4.1Quartz 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

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

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:

 

 

 

LIMIT

 

 

 

 

 

 

 

 

SYMBOL

PARAMETER

MIN.

TYPICAL

MAX.

UNIT

 

 

 

 

 

 

t1

Clock cycle period

39,998

40

40.002

ns

 

 

 

 

 

 

SMSC AN 9.6

 

23

 

Revision 1.0 (08-14-08)

APPLICATION NOTE