xr88c681 Exar Corporation, xr88c681 Datasheet - Page 51

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

Each element of the Timing Control Block is discussed
here:
D.1 Oscillator Circuit:
A crystal oscillator is typically connected externally
across the X1/CLK and X2 pins. The Oscillator Circuit
(within the chip) functions as the load for the resonant
(crystal) oscillator, and buffers the resulting oscillating
Rev. 2.11
ACR[4-6]
X1/CLK
TXCA
TXCB
X2
IP2
Divide by 16
Oscillator
Circuit
Figure 24. Block Diagram of DUART Timing Control Block
Preset Registers
(CTUR, CTLR)
Counter/
Timer
Divide by 16
Generator
Bit Rate
ACR[7]
51
signal, for use by the Bit Rate Generator, and
Counter/Timer. A crystal or TTL signal frequency of
between 2 MHz and 4 MHz is required for proper
operation of the DUART. However, a crystal or TTL signal
frequency of 3.6864 MHz is required for the generation of
standard bit rates by the Bit Rate Generator (See
Table 18).
schematic for the XTAL Oscillator circuitry.
(TXCB)
(RXCA)
IP5
IP4
Figure 25 presents
(TXCA)
(RXCB)
IP3
IP2
XR88C681
MUX
MUX
MUX
MUX
a recommended
32:1
32:1
32:1
32:1
TxCA
RxCA
TxCB
RxCB