UM10462
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© NXP B.V. 2016. All rights reserved.
User manual
Rev. 5.5 — 21 December 2016
269 of 523
NXP Semiconductors
UM10462
Chapter 12: LPC11U3x/2x/1x USART
After reset, synchronous mode is disabled. Synchronous mode is enabled by setting the
SYNC bit. When SYNC is 1, the USART operates as follows:
1. The CSRC bit controls whether the USART sends (master mode) or receives (slave
mode) a serial bit clock on the SCLK pin.
2. When CSRC is 1 selecting master mode, the CSCEN bit selects whether the USART
produces clocks on SCLK continuously (CSCEN=1) or only when transmit data is
being sent on TxD (CSCEN=0).
3. The SSDIS bit controls whether start and stop bits are used. When SSDIS is 0, the
USART sends and samples for start and stop bits as in other modes. When SSDIS is
1, the USART neither sends nor samples for start or stop bits, and each falling edge
on SCLK samples a data bit on RxD into the receive shift register, as well as shifting
the transmit shift register.
The rest of this section provides further details of operation when SYNC is 1.
Data changes on TxD from falling edges on SCLK. When SSDIS is 0, the FES bit controls
whether the USART samples serial data on RxD on rising edges or falling edges on
SCLK. When SSDIS is 1, the USART ignores FES and always samples RxD on falling
edges on SCLK.
The combination SYNC=1, CSRC=1, CSCEN=1, and SSDIS=1 is a difficult operating
mode, because SCLK applies to both directions of data flow and there is no defined
mechanism to signal the receivers when valid data is present on TxD or RxD.
Lacking such a mechanism, SSDIS=1 can be used with CSCEN=0 or CSRC=0 in a mode
similar to the SPI protocol, in which characters are (at least conceptually) “exchanged”
between the USART and remote device for each set of 8 clock cycles on SCLK. Such
operation can be called full-duplex, but the same hardware mode can be used in a
half-duplex way under control of a higher-layer protocol, in which the source of SCLK
toggles it in groups of N cycles whenever data is to be sent in either direction. (N being the
number of bits/character.)
When the LPC11U3x/2x/1x USART is the clock source (CSRC=1), such half-duplex
operation can lead to the rather artificial-seeming requirement of writing a dummy
character to the Transmitter Holding Register in order to generate 8 clocks so that a
character can be received. The CCCLR bit provides a more natural way of programming
half-duplex reception. When the higher-layer protocol dictates that the LPC11U3x/2x/1x
USART should receive a character, software should write the SYNCCTRL register with
6
CCCLR
Continuous clock clear
0
0
CSCEN is under software control.
1
Hardware clears CSCEN after each character is
received.
31:7
-
Reserved. The value read from a reserved bit is not
defined.
NA
Table 256. USART Synchronous mode control register (SYNCCTRL - address 0x4000 8058)
bit description
Bit
Symbol
Value
Description
Reset
value