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XR16L2750IM Datasheet(PDF) 10 Page - Exar Corporation |
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XR16L2750IM Datasheet(HTML) 10 Page - Exar Corporation |
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10 / 49 page  áç áç áç áç XR16L2750 2.25V TO 5.5V DUART WITH 64-BYTE FIFO REV. 1.0.0 10 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. 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. 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 FIGURE 5. EXTERNAL CLOCK CONNECTION FOR EXTENDED DATA RATE FIGURE 6. BAUD RATE GENERATOR AND PRESCALER 2K XTAL1 XTAL2 R1 VCC External Clock vcc gnd XTAL 1 XTAL 2 C rysta l O sc/ B u ffe r M CR B it-7 = 0 (defau lt) M CR B it-7 = 1 D LL a nd D L M R egisters Pre s ca le r D iv ide by 1 Pre s ca le r D iv ide by 4 16X S a m p lin g Ra te Clo c k to T ransm itte r B aud R a te G ene ra to r Logic |
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