Datasheet

UM10398 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2014. All rights reserved.
User manual Rev. 12.3 — 10 June 2014 215 of 547
NXP Semiconductors
UM10398
Chapter 13: LPC111x/LPC11Cxx UART
13.5.15 UART Fractional Divider Register (U0FDR - 0x4000 8028)
The UART Fractional Divider Register (U0FDR) 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 UART disabled making sure that UART is
fully software and hardware compatible with UARTs not equipped with this feature.
The UART baud rate can be calculated as:
(3)
Where UART_PCLK is the peripheral clock, U0DLM and U0DLL are the standard UART
baud rate divider registers, and DIVADDVAL and MULVAL are UART 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 U0FDR should not be modified while transmitting/receiving data or data
may be lost or corrupted.
If the U0FDR 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 200. UART Fractional Divider Register (U0FDR - 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 UART baud rate.
0
7:4 MULVAL Baud rate pre-scaler multiplier value. This field must be greater or
equal 1 for UART 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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