NXP Semiconductors LPC11U3x User Manual

UM10462 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2016. All rights reserved.
User manual Rev. 5.5 — 21 December 2016 260 of 523
NXP Semiconductors
UM10462
Chapter 12: LPC11U3x/2x/1x USART
12.5.14 USART Fractional Divider Register
The USART Fractional Divider Register (FDR) controls the clock pre-scaler for the baud
rate generation and can be read and written at the user’s discretion. This pre-scaler takes
the APB clock and generates an output clock according to the specified fractional
requirements.
Important: If the fractional divider is active (DIVADDVAL > 0) and DLM = 0, the value of
the DLL register must be 3 or greater.
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:
(3)
Where UART_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.
Table 247. USART Fractional Divider Register (FDR - address 0x4000 8028) 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 U0DLM U0DLL+ 1
DivAddVal
MulVal
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NXP Semiconductors LPC11U3x Specifications

General

Brand	NXP Semiconductors
Model	LPC11U3x
Category	Microcontrollers
Language	English