XR88C681J-F Datasheet, PDF (52/101 Pages) Exar Corporation – Two Full Duplex, Independent Channels

XR88C681
C1: 10pF + (Stray < 5pF)
C2: 10pF + (Stray < 5pF)
R1: 100ohm
R2: 100ohm
R1
X1
C1
XR88C681
C2
R2
X2
3.6864MHz
Parallel Resonant Crystal
Figure 25. A Recommended Schematic for the XTAL Oscillator Circuitry
Note: The user also has an option to drive the Oscillator Circuit with a TTL input signal, in lieu of using a crystal oscillator. If this
approach is used, the TTL must be driven into the X1/CLK pin, and the X2 pin must be left floating.
If the user desires to run numerous DUARTs from a single crystal oscillator, Figure 26 presents an approach and the
necessary circuitry to accomplish this objective.
XR88C681
X1
X2
Y1
3.684MHz
74HC14
To X1 inputs of
other DUARTs
Figure 26. A Recommended Schematic to Drive Multiple DUARTs From the Same Crystal Oscillator.
Note: The user is urged not to use the 74LS14 Schmitt Trigger Inverter in lieu of the 74HC14 device. The input of the 74LS14 tends to
load down the oscillating signal from the DUART, to the point that the Schmitt Trigger inverter can no longer change state or
respond to the oscillator signal.
Rev. 2.11
52