UM10237_2 © NXP B.V. 2008. All rights reserved.
User manual Rev. 02 — 19 December 2008 437 o f 792
NXP Semiconductors UM10237
Chapter 16: LPC24XX Universal Asynchronous Receiver/Transmitter
4.12 UARTn Fractional Divider Register (U0FDR - 0xE000 C028, U2FDR - 0xE007 8028, U3FDR - 0xE007 C028)
The UART0/2/3 Fractional Divider Register (U0/2/3FDR) 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 UART0/2/3 disabled making sure that
UART0/2/3 is fully software and hardware compatible with UARTs not equipped with this
feature.
UART0/2/3 baudrate can be calculated as (n = 0/2/3):
(2)
Where PCLK is the peripheral clock, U0/2/3DLM and U0/2/3DLL are the standard
UART0/2/3 baud rate divider registers, and DIVADDVAL and MULVAL are UART0/2/3
fractional baudrate generator specific parameters.
128 × TPCLK
1316 × TPCLK
1432 × TPCLK
1564 × TPCLK
1 6 128 × TPCLK
1 7 256 × TPCLK
Table 391: IrDA Pulse Width
FixPulseEn PulseDiv IrDA Transmitter Pulse width (µs)
Table 392: UARTn Fractional Divider Register (U0FDR - address 0xE000 C028,
U2FDR - 0xE007 8028, U3FDR - 0xE007C0 28) bit description
Bit Function Value Description Reset
value
3:0 DIVADDVAL 0 Baud-rate generation pre-scaler divisor value. If this field is
0, fractional baud-rate generator will not impact the UARTn
baudrate.
0
7:4 MULVAL 1 Baud-rate pre-scaler multiplier value. This field must be
greater or equal 1 for UARTn to operate properly,
regardless of whether the fractional baud-rate generator is
used or not.
1
31:8 - NA Reserved, user software should not write ones to reserved
bits. The value read from a reserved bit is not defined. 0
UARTnbaudrate PCLK
16 256 Un DLM×UnDLL+()×1DivAddVal
MulVal
-----------------------------
+
⎝⎠
⎛⎞
×
----------------------------------------------------------------------------------------------------------------------------------
=