
SC16IS740_750_760
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NXP B.V. 2011. All rights reserved.
Product data sheet
Rev. 7 — 9 June 2011
18 of 63
NXP Semiconductors
SC16IS740/750/760
Single UART with I2C-bus/SPI interface, 64-byte FIFOs, IrDA SIR
7.8 Programmable baud rate generator
The SC16IS740/750/760 UART contains a programmable baud rate generator that takes
any clock input and divides it by a divisor in the range between 1 and (216 –1). An
additional divide-by-4 prescaler is also available and can be selected by MCR[7], as
shown in
Figure 15. The output frequency of the baud rate generator is 16
the baud
rate. The formula for the divisor is given in
Equation 1:
(1)
where:
prescaler = 1, when MCR[7] is set to ‘0’ after reset (divide-by-1 clock selected)
prescaler = 4, when MCR[7] is set to ‘1’ after reset (divide-by-4 clock selected).
Remark: The default value of prescaler after reset is divide-by-1.
Figure 15 shows the internal prescaler and baud rate generator circuitry.
DLL and DLH must be written to in order to program the baud rate. DLL and DLH are the
least significant and most significant byte of the baud rate divisor. If DLL and DLH are both
zero, the UART is effectively disabled, as no baud clock will be generated.
Remark: The programmable baud rate generator is provided to select both the transmit
and receive clock rates.
Table 7 and
Table 8 show the baud rate and divisor correlation for crystal with frequency
1.8432 MHz and 3.072 MHz, respectively. The crystal’s frequency tolerance should be
selected as such to keep the baud rate error to be below 1 % for reliable operation with
other UARTs. Crystals with
100 ppm is generally recommended.
Figure 16 shows the crystal clock circuit reference.
Fig 15. Prescaler and baud rate generator block diagram
divisor
XTAL1 crystal input frequency
prescaler
-----------------------------------------------------------------------------------
desired baud rate
16
-----------------------------------------------------------------------------------------
=
BAUD RATE
GENERATOR
LOGIC
MCR[7] = 1
MCR[7] = 0
PRESCALER
LOGIC
(DIVIDE-BY-1)
INTERNAL
OSCILLATOR
LOGIC
002aaa233
XTAL1
XTAL2
input clock
PRESCALER
LOGIC
(DIVIDE-BY-4)
reference
clock
internal
baud rate
clock for
transmitter
and receiver