XR17D158_05 Datasheet, PDF (26/73 Pages) Exar Corporation – UNIVERSAL (3.3V AND 5V) PCI BUS OCTAL UART

XR17D158
UNIVERSAL (3.3V AND 5V) PCI BUS OCTAL UART
xr
REV. 1.2.2
3.0 CRYSTAL OSCILLATOR / BUFFER
The D158 includes an on-chip oscillator (XTAL1 and XTAL2). The crystal oscillator provides the system clock
to the Baud Rate Generators (BRG) in each of the 8 UARTs, the 16-bit general purpose timer/counter and
internal logics. XTAL1 is the input to the oscillator or external clock buffer input with XTAL2 pin being the
output. See Programmable Baud Rate Generator in the UART section for programming details.
The on-chip oscillator is designed to use an industry standard microprocessor crystal (parallel resonant with
10-22 pF capacitance load, 100ppm) connected externally between the XTAL1 and XTAL2 pins (see Figure 9).
Alternatively, an external clock can be connected to the XTAL1 pin to clock the internal 8 baud rate generators
for standard or custom rates. However, for external clock frequencies greater than 24MHz, a 2K pull-up may be
necessary on the XTAL2 output (see Figure 10). Typically, the oscillator connections are shown in Figure 9.
For further reading on oscillator circuit please see application note DAN108 on EXAR’s web site.
FIGURE 9. TYPICAL OSCILLATOR CONNECTIONS
R=300K to 400K
XTAL1 14.7456 XTAL2
MHz
C1
22-47pF
C2
22-47pF
FIGURE 10. EXTERNAL CLOCK CONNECTION FOR EXTENDED DATA RATE
External Clock
vcc
gnd
VCC
R1
2K
XTAL1
XTAL2
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