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XR16L2751IM Datasheet(PDF) 11 Page - Exar Corporation |
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XR16L2751IM Datasheet(HTML) 11 Page - Exar Corporation |
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11 / 52 page  XR16L2751 áç áç áç áç 2.25V TO 5.5V DUART WITH 64-BYTE FIFO AND POWERSAVE REV. 1.0.0 11 2.9 Crystal Oscillator or External Clock Input The 2751 includes an on-chip oscillator (XTAL1 and XTAL2) to produce a clock for both UART sections in the device. The CPU data bus does not require this clock for bus operation. The crystal oscillator provides a system clock to the Baud Rate Generators (BRG) section found in each of the UART. XTAL1 is the input to the oscillator or external clock buffer input with XTAL2 pin being the output. Please note that the input XTAL1 is not 5V tolerant and so the maximum voltage at the pin should be VCC. For programming details, see “Programmable Baud Rate Generator.” FIGURE 4. TYPICAL OSCILLATOR CONNECTIONS 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 100ppm frequency tolerance) connected externally between the XTAL1 and XTAL2 pins (see Figure 5). 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 CLKSEL hardware pin or 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 and can override the CLKSEL pin following reset. The clock output of the FIGURE 5. EXTERNAL CLOCK CONNECTION FOR EXTENDED DATA RATE C1 22-47pF C2 22-47pF Y1 1.8432 MHz to 24 MHz R1 0-120 (Optional) R2 500K - 1M XTAL1 XTAL2 2K XT A L 1 XT A L 2 R1 VC C E x te r nal C loc k vcc gnd |
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