UM10462
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User manual
Rev. 5.5 — 21 December 2016
264 of 523
NXP Semiconductors
UM10462
Chapter 12: LPC11U3x/2x/1x USART
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 for 7 clocks/bit and
OSFrac to 011 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” (372 minus
one).
12.5.16 USART Transmit Enable Register
In addition to being equipped with full hardware flow control (auto-cts and auto-rts
mechanisms described above), TER enables implementation of software flow control.
When TxEn = 1, the USART transmitter will keep sending data as long as they are
available. As soon as TxEn becomes 0, USART transmission will stop.
describes how to use TxEn bit in order to achieve hardware flow
control, it is strongly suggested to let the USART hardware implemented auto flow control
features take care of this and limit the scope of TxEn to software flow control.
describes how to use TXEn bit in order to achieve software flow control.
Table 249. USART Oversampling Register (OSR - address 0x4000 802C) 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