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XR16C2850IJ Datasheet(PDF) 11 Page - Exar Corporation |
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XR16C2850IJ Datasheet(HTML) 11 Page - Exar Corporation |
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11 / 51 page  xr XR16C2850 REV. 2.1.3 2.97V TO 5.5V DUAL UART WITH 128-BYTE FIFOS 11 Typical oscillator connections are shown in Figure 4. For further reading on oscillator circuit please see application note DAN108 on EXAR’s web site. 2.9 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 5) it can extend its operation up to 50 MHz (3.125 Mbps serial data rate and 16X sampling) at room temperature and 5.0V. FIGURE 5. EXTERNAL CLOCK CONNECTION FOR EXTENDED DATA RATE Each UART also has their own prescaler along with the BRG. 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 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. 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 sampling clock rate mode to double the operating data rate. When using a non-standard data rate crystal or external clock, the divisor value can be calculated for DLL/DLM with the following equation. FIGURE 6. BAUD RATE GENERATOR AND PRESCALER 2K XTAL1 XTAL2 R1 VCC External Clock vcc gnd XTAL1 XTAL2 Crystal Osc/ Buffer MCR Bit-7=0 (default) MCR Bit-7=1 DLL and DLM Registers Prescaler Divide by 1 Prescaler Divide by 4 16X Sampling Rate Clock to Transmitter Baud Rate Generator Logic |
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