XR16C850CM-F Datasheet, PDF (14/56 Pages) Exar Corporation – 2.97V TO 5.5V UART WITH 128-BYTE FIFO

XR16C850
2.97V TO 5.5V UART WITH 128-BYTE FIFO
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REV. 2.3.1
2.11 Programmable Baud Rate Generator
The UART has its own Baud Rate Generator (BRG) with a prescaler for the transmitter. The prescaler is
controlled by 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. 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 clock of the serial data
rate. The sampling clock is used by the transmitter for data bit shifting and receiver for data sampling. The BRG
divisor (DLL and DLM registers) defaults to a random value upon power up or a reset. Therefore, the BRG
must be programmed during initialization to the operating data rate.
FIGURE 8. BAUD RATE GENERATOR
XTAL1
XTAL2
Crystal
Osc/
Buffer
During
Initialization or
Reset
CLKSEL = VCC
CLKSEL = GND
Prescaler
Divide by 1
Prescaler
Divide by 4
DLL and DLM
Registers
MCR Bit-7=0
(default)
Baud Rate
Generator
Logic
MCR Bit-7=1
16X
Sampling
Rate Clock to
Transmitter
and Receiver
Programming the Baud Rate Generator Registers DLM and DLL provides the capability of selecting the
operating data rate. Table 4 shows the standard data rates available with a 14.7456 MHz crystal or external
clock at 16X clock 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.
divisor (decimal) = (XTAL1 clock frequency / prescaler) / (serial data rate x 16)
TABLE 4: TYPICAL DATA RATES WITH A 14.7456 MHZ CRYSTAL OR EXTERNAL CLOCK
OUTPUT Data Rate OUTPUT Data Rate DIVISOR FOR 16x DIVISOR FOR 16x
MCR Bit-7=1
MCR Bit-7=0 Clock (Decimal) Clock (HEX)
DLM
PROGRAM
VALUE (HEX)
DLL
PROGRAM
VALUE (HEX)
DATA RATE
ERROR (%)
100
400
2304
900
09
00
0
600
2400
384
180
01
80
0
1200
4800
192
C0
00
C0
0
2400
9600
96
60
00
60
0
4800
19.2k
48
30
00
30
0
9600
38.4k
24
18
00
18
0
19.2k
76.8k
12
0C
00
0C
0
38.4k
153.6k
6
06
00
06
0
57.6k
230.4k
4
04
00
04
0
115.2k
460.8k
2
02
00
02
0
230.4k
921.6k
1
01
00
01
0
14