UM11029
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User manual
Rev. 1.0 — 16 June 2017
304 of 515
NXP Semiconductors
UM11029
Chapter 17: LPC84x USART0/1/2/3/4
17.7.3 Synchronous mode
Remark:
Synchronous mode transmit and receive operate at the incoming clock rate in
slave mode and the BRG selected rate (not divided by 16) in master mode.
17.7.4 Flow control
The USART supports both hardware and software flow control.
17.7.4.1 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.
If connected to a pin, and if enabled to do so, the CTS input can allow an external device
to throttle the USART transmitter.
shows an overview of RTS and CTS within the USART.
17.7.4.2 Software flow control
Software flow control could include XON / XOFF flow control, or other mechanisms. these
are supported by the ability to check the current state of the CTS input, and/or have an
interrupt when CTS changes state (via the CTS and DELTACTS bits, respectively, in the
STAT register), and by the ability of software to gracefully turn off the transmitter (via the
TXDIS bit in the CTL register).
17.7.5 Autobaud function
The autobaud functions attempts to measure the start bit time of the next received
character. For this to work, the measured character must have a 1 in the least significant
bit position, so that the start bit is bounded by a falling and rising edge. The measurement
is made using the current clocking settings, including the oversampling configuration. The
result is that a value is stored in the BRG register that is as close as possible to the correct
setting for the sampled character and the current clocking settings. The sampled
character is provided in the RXDAT and RXDATSTAT registers, allowing software to
double-check for the expected character.
Fig 26. Hardware flow control using RTS and CTS
Transmitter
Receiver
CFG
[CTSEN]
Un_CTS
Un_RTS
change detect
STAT [CTS]
STAT [DELTACTS]
CFG [LOOP]
0
1