xr88c681 Exar Corporation, xr88c681 Datasheet - Page 52

• Manufacturer Part Number: xr88c681
• Description: Cmos Dual Channel Uart Duart
• Manufacturer: Exar Corporation

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
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.
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
Figure 26. A Recommended Schematic to Drive Multiple DUARTs From the Same Crystal Oscillator.
Rev. 2.11
approach is used, the TTL must be driven into the X1/CLK pin, and the X2 pin must be left floating.
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.
Figure 25. A Recommended Schematic for the XTAL Oscillator Circuitry
C1
C2
3.6864MHz
X1
Parallel Resonant Crystal
C1: 10pF + (Stray < 5pF)
C2: 10pF + (Stray < 5pF)
R1: 100ohm
R2: 100ohm
XR88C681
3.684MHz
Y1
52
X2
R1
R2
X1
X2
XR88C681
74HC14
To X1 inputs of
other DUARTs