XR16L2750_05 Datasheet, PDF (10/48 Pages) Exar Corporation – 2.25V TO 5.5V DUART WITH 64-BYTE FIFO

XR16L2750
2.25V TO 5.5V DUART WITH 64-BYTE FIFO
xr
REV. 1.2.1
The on-chip oscillator is designed to use an industry standard microprocessor crystal (parallel resonant,
fundamental frequency with 10-22 pF capacitance load, ESR of 20-120 ohms and 100 ppm frequency
tolerance) connected externally between the XTAL1 and XTAL2 pins (see Figure 4). The programmable Baud
Rate Generator is capable of operating with a crystal oscillator 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 5) it can
extend its operation up to 50 MHz (6.25 Mbps serial data rate) at 5V with an 8X sampling rate.
FIGURE 5. EXTERNAL CLOCK CONNECTION FOR EXTENDED DATA RATE
External Clock
vcc
gnd
VCC
R1
2K
XTAL1
XTAL2
For further reading on the oscillator circuit please see the Application Note DAN108 on the EXAR web site at
http://www.exar.com.
2.10 Programmable Baud Rate Generator
Each UART has its own Baud Rate Generator (BRG) with a prescaler. 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 clock output of the prescaler goes to the BRG. The BRG further divides this clock by a
programmable divisor between 1 and (2¹⁶ -1) to obtain a 16X sampling rate clock of the serial data rate. The
sampling rate clock is used by the transmitter for data bit shifting and receiver for data sampling. The BRG
divisor defaults to the maximum baud rate (DLL = 0x01 and DLM = 0x00) upon power up.
FIGURE 6. BAUD RATE GENERATOR AND PRESCALER
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
Sampling
Rate Clock to
Transmitter
Programming the Baud Rate Generator Registers DLM and DLL provides the capability of selecting the
operating data rate. Table 5 shows the standard data rates available with a 14.7456 MHz crystal or external
clock at 16X sampling rate clock rate. A 16X sampling clock is typically used. However, user can select the 8X
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