á
3.3V AND 5V DUART WITH 128-BYTE FIFO
XR16C2852
REV. 2.0.0
8
2.8
The INTA and INTB interrupt output changes accord-
ing to the operating mode and enahnced features set-
INTA
AND
INTB O
UPUTS
up. Table 3 and 4 summarize the operating behavior
for the transmitter and receiver. Also see Figures 18
through 23.
2.9
The 2852 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. For programming details,
see “Programmable Baud Rate Generator.”
C
RYSTAL
O
SCILLATOR
OR
E
XT
. C
LOCK
I
NPUT
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 20-80 ohms and 100ppm frequency tol-
erance) connected externally between the XTAL1 and
XTAL2 pins (see Figure 4). Alternatively, an external
clock can be connected to the XTAL1 pin to clock the
internal baud rate generator for standard or custom
rates. Typical oscillator connections are shown in
Figure 4. For further reading on oscillator circuit
please see application note DAN108 on EXAR’s web
site.
2.10 P
ROGRAMMABLE
B
AUD
R
ATE
G
ENERATOR
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 re-
sistor on XTAL2 pin (as shown in Figure 5) it can ex-
tend its operation up to 50 MHz (3.125 Mbps serial
data rate) at room temperature and 5.0V.
T
ABLE
3: INTA
AND
INTB P
INS
O
PERATION
FOR
T
RANSMITTER
Auto RS485
Mode
FCR B
IT
-0 = 0
(FIFO D
ISABLED
)
FCR B
IT
-0 = 1
(FIFO E
NABLED
)
INTA/B Pin
NO
0 = a byte in THR
1 = THR empty
0 = FIFO above trigger level
1 = FIFO below trigger level or FIFO empty
INTA/B Pin
YES
0 = a byte in THR
1 = transmitter empty
0 = FIFO above trigger level
1 = FIFO below trigger level or transmitter empty
T
ABLE
4: INTA
AND
INTB P
IN
O
PERATION
F
OR
R
ECEIVER
FCR B
IT
-0 = 0
(FIFO D
ISABLED
)
FCR B
IT
-0 = 1
(FIFO E
NABLED
)
INTA/B Pin
0 = no data
1 = 1 byte
0 = FIFO below trigger level
1 = FIFO above trigger level
F
IGURE
4. T
YPICAL
OSCILLATOR
CONNECTIONS
C1
22-47 pF
C2
22-47 pF
Y1
1.8432 MHz
to
24 MHz
R1
0-120
(Optional)
R2
500 Κ 1 Μ
XTAL1
XTAL2