UM10503
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© NXP B.V. 2012. All rights reserved.
User manual
Rev. 1.3 — 6 July 2012
964 of 1269
NXP Semiconductors
UM10503
Chapter 37: LPC43xx USART0_2_3
This register controls the clock pre-scaler for the baud rate generation. The reset value of
the register keeps the fractional capabilities of USART disabled making sure that USART
is fully software and hardware compatible with USARTs not equipped with this feature.
The USART baud rate can be calculated as:
(6)
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.
37.6.13 USART Oversampling Register
In most applications, the USART samples received data 16 times in each nominal bit time
and sends bits that are 16 input clocks wide. This register allows software to control the
ratio between the input clock and bit clock. Oversampling is required for smart card mode
and provides an alternative to fractional division for other modes.
Table 836. USART Fractional Divider Register (FDR - addresses 0x4008 1028 (USART0),
0x400C 1028 (USART2), 0x400C 2028 (USART3)) bit description
Bit
Function
Description
Reset
value
3:0
DIVADDVAL
Baud rate generation pre-scaler divisor value.
If this field is 0, fractional baud rate generator will not impact the
USART baud rate.
0
7:4
MULVAL
Baud rate pre-scaler multiplier value.
This field must be greater or equal 1 for USART to operate
properly, regardless of whether the fractional baud rate
generator is used or not.
1
31:8
-
Reserved, user software should not write ones to reserved bits.
The value read from a reserved bit is not defined.
0
UART
baudrate
PCLK
16
256
DLM
DLL
+
1
DivAddVal
MulVal
-----------------------------
+
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