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UM10850
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User manual
Rev. 2.4 — 13 September 2016
276 of 464
NXP Semiconductors
UM10850
Chapter 21: LPC5410x USARTs (USART0/1/2/3)
data bit must be reduced to one. Finally, special clocking has to be used for individual bit
times because 32 kHz is not particularly close to an even multiple of any standard baud
rate.
When 32 kHz mode is enabled, clocking comes from the RTC oscillator. The FRG is
bypassed, and the BRG can be used to divide down the default 9600 baud to lower rates.
Other adaptations required to make the USART work for rates up to 9600 baud are done
internally. Rate error will be less than one half percent in this mode, provided the RTC
oscillator is operating at the intended frequency of 32.768 kHz.
21.7.2 DMA
A DMA request is provided for each USART direction, and can be used in lieu of interrupts
for transferring data by configuring the DMA controller appropriately. The DMA controller
provides an acknowledgement signal that clears the related request when it completes
handling a that request. The transmitter DMA request is asserted when the transmitter
can accept more data. The receiver DMA request is asserted when received data is
available to be read.
When DMA is used to perform USART data transfers, other mechanisms can be used to
generate interrupts when needed. For instance, completion of the configured DMA
transfer can generate an interrupt from the DMA controller. Also, interrupts for special
conditions, such as a received break, can still generate useful interrupts.
21.7.3 Synchronous mode
In synchronous mode, a master generates a clock as defined by the clock selection and
BRG, which is used to transmit and receive data. As a slave, the external clock is used to
transmit and receive data. There is no overclocking in either case.
21.7.4 Flow control
The USART supports both hardware and software flow control.
21.7.4.1 Hardware flow control
The USART supports hardware flow control using RTS and/or CTS signalling. If RTS is
configured to appear on a device pin so that it can be sent to an external device, it
indicates to an external device the ability of the receiver to receive more data. It can also
be used internally to throttle the transmitter from the receiver, which can be especially
useful if loopback mode is enabled.
If connected to a pin, and if enabled to do so, the CTS input can allow an external device
to throttle the USART transmitter. Both internal and external CTS can be used separately
or together.
shows an overview of RTS and CTS within the USART.