XR19L400 Datasheet, PDF (9/45 Pages) Exar Corporation – SINGLE CHANNEL INTEGRATED UART AND RS-485 TRANSCEIVER

PRELIMINARY
XR19L400
REV. P1.0.0
SINGLE CHANNEL INTEGRATED UART AND RS-485 TRANSCEIVER
bit shifting and receiver for data sampling. The BRG divisor (DLL, DLM and DLD registers) defaults to the value
of ’1’ (DLL = 0x01, DLM = 0x00 and DLD = 0x00) upon reset. Therefore, the BRG must be programmed during
initialization to the operating data rate. The DLL and DLM registers provide the integer part of the divisor and
the DLD register provides the fractional part of the dvisior. Only the four lower bits of the DLD are implemented
and they are used to select a value from 0 (for setting 0000) to 0.9375 or 15/16 (for setting 1111). Programming
the Baud Rate Generator Registers DLL, DLM and DLD provides the capability for selecting the operating data
rate. Table 3 shows the standard data rates available with a 24MHz crystal or external clock at 16X clock rate.
If the pre-scaler is used (MCR bit-7 = 1), the output data rate will be 4 times less than that shown in Table 3. At
8X sampling rate, these data rates would double. Also, when using 8X sampling mode, please note that the bit-
time will have a jitter (+/- 1/16) whenever the DLD is non-zero and is an odd number. When using a non-
standard data rate crystal or external clock, the divisor value can be calculated with the following equation(s):
Required Divisor (decimal) = (XTAL1 clock frequency / prescaler) / (serial data rate x 16), with 16X mode EMSR[7] = 1
Required Divisor (decimal) = (XTAL1 clock frequency / prescaler / (serial data rate x 8), with 8X mode EMSR[7] = 0
The closest divisor that is obtainable in the L400 can be calculated using the following formula:
ROUND( (Required Divisor - TRUNC(Required Divisor) )*16)/16 + TRUNC(Required Divisor), where
DLM = TRUNC(Required Divisor) >> 8
DLL = TRUNC(Required Divisor) & 0xFF
DLD = ROUND( (Required Divisor-TRUNC(Required Divisor) )*16)
In the formulas above, please note that:
TRUNC (N) = Integer Part of N. For example, TRUNC (5.6) = 5.
ROUND (N) = N rounded towards the closest integer. For example, ROUND (7.3) = 7 and ROUND (9.9) = 10.
A >> B indicates right shifting the value ’A’ by ’B’ number of bits. For example, 0x78A3 >> 8 = 0x0078.
FIGURE 5. BAUD RATE GENERATOR
XTAL1
XTAL2
Crystal
Osc/
Buffer
Prescaler
Divide by 1
Prescaler
Divide by 4
DLL, DLM and DLD
Registers
MCR Bit-7=0
(default)
Fractional Baud
Rate Generator
Logic
16X or 8X
Sampling
Rate Clock
to Transmitter
and Receiver
MCR Bit-7=1
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