UM10462 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2016. All rights reserved.
User manual Rev. 5.5 — 21 December 2016 271 of 523
NXP Semiconductors
UM10462
Chapter 12: LPC11U3x/2x/1x USART
While the receiver is enabled (RS485CTRL bit 1 = ‘0’), all bytes received will be accepted
and stored in the RXFIFO until an address byte which does not match the
RS485ADRMATCH value is received. When this occurs, the receiver will be automatically
disabled in hardware (RS485CTRL bit 1 will be set), The received non-matching address
character will not be stored in the RXFIFO.
RS-485/EIA-485 Auto Direction Control
RS485/EIA-485 mode includes the option of allowing the transmitter to automatically
control the state of the DIR pin as a direction control output signal.
Setting RS485CTRL bit 4 = ‘1’ enables this feature.
Keep RS485CTRL bit 3 zero so that direction control, if enabled, will use the RTS
pin.
When Auto Direction Control is enabled, the selected pin will be asserted (driven LOW)
when the CPU writes data into the TXFIFO. The pin will be de-asserted (driven HIGH)
once the last bit of data has been transmitted. See bits 4 and 5 in the RS485CTRL
register.
The RS485CTRL bit 4 takes precedence over all other mechanisms controlling the
direction control pin with the exception of loopback mode.
RS485/EIA-485 driver delay time
The driver delay time is the delay between the last stop bit leaving the TXFIFO and the
de-assertion of RTS
. This delay time can be programmed in the 8-bit RS485DLY register.
The delay time is in periods of the baud clock. Any delay time from 0 to 255 bit times may
be used.
RS485/EIA-485 output inversion
The polarity of the direction control signal on the RTS
(or DTR) pins can be reversed by
programming bit 5 in the RS485CTRL register. When this bit is set, the direction control
pin will be driven to logic 1 when the transmitter has data waiting to be sent. The direction
control pin will be driven to logic 0 after the last bit of data has been transmitted.
12.6.2 Smart card mode
Figure 29 shows a typical asynchronous smart card application.
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