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UM10503
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© NXP B.V. 2015. All rights reserved.
User manual
Rev. 2.1 — 10 December 2015
1144 of 1441
NXP Semiconductors
UM10503
Chapter 40: LPC43xx/LPC43Sxx USART0_2_3
40.7.3 Baud rate calculation in asynchronous mode
USART can operate with or without using the Fractional Divider. In real-life applications it
is likely that the desired baud rate can be achieved using several different Fractional
Divider settings. The following algorithm illustrates one way of finding a set of DLM, DLL,
MULVAL, and DIVADDVAL values. Such set of parameters yields a baud rate with a
relative error of less than 1.1% from the desired one.
The USART baud rate can be calculated as:
(8)
Where USART_PCLK is the peripheral clock, DLM and DLL are the standard USART
baud rate divider registers, and DIVADDVAL and MULVAL are USART fractional baud
rate generator specific parameters.
The value of MULVAL and DIVADDVAL should comply to the following conditions:
1. 1
MULVAL
15
2. 0
DIVADDVAL
14
3. DIVADDVAL< MULVAL
The value of the FDR should not be modified while transmitting/receiving data or data may
be lost or corrupted.
If the FDR register value does not comply to these two requests, then the fractional divider
output is undefined. If DIVADDVAL is zero then the fractional divider is disabled, and the
clock will not be divided.
UART
baudrate
PCLK
16
256
DLM
DLL
+
1
DivAddVal
MulVal
-----------------------------
+
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