UM10503
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© NXP B.V. 2012. All rights reserved.
User manual
Rev. 1.3 — 6 July 2012
965 of 1269
NXP Semiconductors
UM10503
Chapter 37: LPC43xx USART0_2_3
Example:
For a baud rate of 3.25 Mbps with a 24 MHz USART clock frequency, the ideal
oversampling ratio is 24/3.25 or 7.3846. Setting OSINT to 0110 (binary) for 7 clocks/bit
and OSFrac to 011 (binary) for 0.375 clocks/bit, results in an oversampling ratio of 7.375.
In Smart card mode, OSInt is extended by FDINT. This extends the possible oversampling
to 2048, as required to support ISO 7816-3. Note that this value can be exceeded when
D<0, but this is not supported by the USART. When Smart card mode is enabled, the
initial value of OSINT and FDINT should be programmed as 00101110011 (binary) (372
minus one).
37.6.14 USART Half-duplex enable register
Remark:
The HDEN register should be disabled when in smart card mode (smart card by
default runs in half-duplex mode).
After reset the USART will be in full-duplex mode, meaning that both TX and RX work
independently. After setting the HDEN bit, the USART will be in half-duplex mode. In this
mode, the USART ensures that the receiver is locked when idle, or will enter a locked
state after having received a complete ongoing character reception. Line conflicts must be
handled in software. The behavior of the USART is unpredictable when data is presented
for reception while data is being transmitted.
For this reason, the value of the HDEN register should not be modified while sending or
receiving data, or data may be lost or corrupted.
Table 837. USART Oversampling Register (OSR - addresses 0x4008 102C (USART0), 0x400C
102C (USART2), 0x400C 20402C (USART3)) bit description
Bit
Symbol
Description
Reset
value
0
-
Reserved, user software should not write ones to reserved bits.
The value read from a reserved bit is not defined.
NA
3:1
OSFRAC
Fractional part of the oversampling ratio, in units of 1/8th of an
input clock period. (001 = 0.125, ..., 111 = 0.875)
0
7:4
OSINT
Integer part of the oversampling ratio, minus 1. The reset values
equate to the normal operating mode of 16 input clocks per bit
time.
0xF
14:8
FDINT
In Smart Card mode, these bits act as a more-significant extension
of the OSint field, allowing an oversampling ratio up to 2048 as
required by ISO7816-3. In Smart Card mode, bits 14:4 should
initially be set to 371, yielding an oversampling ratio of 372.
0
31:15 -
Reserved, user software should not write ones to reserved bits.
The value read from a reserved bit is not defined.
NA