XR16L784_08 Datasheet, PDF (8/51 Pages) Exar Corporation – HIGH PERFORMANCE 2.97V TO 5.5V QUAD UART

XR16L784
HIGH PERFORMANCE 2.97V TO 5.5V QUAD UART
REV. 1.2.3
2.6 Programmable Baud Rate Generator
A single Baud Rate Generator (BRG) is provided for the transmitter and receiver, allowing independent TX/RX
channel control. The programmable Baud Rate Generator is capable of operating with a crystal frequency of
up to 24 MHz. However, with an external clock input on XTAL1 pin and a 2K ohms pull-up resistor on XTAL2
pin (as shown in Figure 4) it can extend its operation up to 50 MHz (3.125 Mbps serial data rate) at room
temperature and 5.0V.
FIGURE 4. EXTERNAL CLOCK CONNECTION FOR EXTENDED DATA RATE
External Clock
vcc
gnd
VCC
R1
2K
XTAL1
XTAL2
Each UART has its own Baud Rate Generator (BRG) with a prescaler for the transmitter and receiver. The
prescaler is controlled by a software bit in the MCR register. The MCR register bit-7 sets the prescaler to divide
the input crystal or external clock by 1 or 4. The output of the prescaler clocks to the BRG. The BRG further
divides this clock by a programmable divisor between 1 and (2¹⁶ -1) to obtain a 16X or 8X sampling clock of
the serial data rate. The sampling clock is used by the transmitter for data bit shifting and receiver for data
sampling. The BRG divisor (DLL and DLM registers) defaults to a random value upon power up. Therefore, the
BRG must be programmed during initialization to the operating data rate.
FIGURE 5. BAUD RATE GENERATOR
To Other
Channels
XTAL1
XTAL2
Crystal
Osc/
Buffer
Prescaler
Divide by 1
Prescaler
Divide by 4
DLL and DLM
Registers
MCR Bit-7=0
(default)
Baud Rate
Generator
Logic
MCR Bit-7=1
16X or 8X
Sampling
Rate Clock to
Transmitter
and Receiver
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