XR16C850_05 Datasheet, PDF (13/56 Pages) Exar Corporation – 2.97V TO 5.5V UART WITH 128-BYTE FIFO

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REV. 2.3.1
XR16C850
2.97V TO 5.5V UART WITH 128-BYTE FIFO
2.9 Interrupts
The output function of interrupt outputs change according to the operating bus type. During the Intel Bus Mode,
the INT output will always be active high and MCR bit-3 will have no effect on the INT output pin. In the PC
Mode, the IRQ outputs are in three-state mode unless MCR bit-3 and S3 are both a logic 1. Table 3
summarizes its behavior in Intel and PC mode of operation.
TABLE 3: INTERRUPT OUTPUT FUNCTIONS
BUS
MODE
MCR
BIT-3
S3
(PC MODE
ONLY)
INTERRUPT OUTPUT
(INT OR IRQ)
Intel
X
X
Active High
PC
0
0
Three-State
0
1
Three-State
1
0
Three-State
1
1
Active High
FIGURE 7. TYPICAL OSCILLATOR CONNECTIONS
XTAL1
C1
22-47pF
XTAL2
R2
500K - 1M
R1
0-120
(Optional)
Y1 ^{1.8432 MHz}
to
24 MHz
C2
22-47pF
2.10 Crystal Oscillator or External Clock
The 850 includes an on-chip oscillator (XTAL1 and XTAL2). The crystal oscillator provides the system clock to
the Baud Rate Generators (BRG) in the UART. XTAL1 is the input to the oscillator or external clock buffer input
with XTAL2 pin being the output. For programming details, see “Section 2.11, Programmable Baud Rate
Generator” on page 14. To use the same clock for the receiver as used with the transmiter of the UART in the
Intel bus mode, the BAUDCLK pin must be connected to the RCLK pin external to the UART.
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 7). Alternatively, an external
clock can be connected to the XTAL1 pin to clock the internal baud rate generator for standard or custom rates.
Typically, the oscillator connections are shown in Figure 7. For further reading on oscillator circuit please see
application note DAN108 on EXAR’s web site.
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