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UM10316_0
© NXP B.V. 2008. All rights reserved.
User manual
Rev. 00.06 — 17 December 2008
292 of 571
NXP Semiconductors
UM10316
Chapter 19: LPC29xx Universal Asynchronous Receiver/Transmitter
4.16 UART0 RS485 Address Match register
4.17 UART1 RS-485 Delay value register
The user may program the 8-bit RS485DLY register with a delay between the last stop bit
leaving the TXFIFO and the de-assertion of RTS (or DTR). This delay time is in periods of
the baud clock. Any delay time from 0 to 255 bit times may be programmed.
4.18 RS-485 modes of operation
RS-485 Normal Multidrop Mode (NMM)
Setting the RS485CTRL bit 0 enables this mode. In this mode, an address is detected
when a received byte causes the UART to set the parity error and generate an interrupt.
If the receiver is DISABLED (RS485CTRL bit 1 = ‘1’) any received data bytes will be
ignored and will not be stored in the RXFIFO. When an address byte is detected (parity
bit = ‘1’) it will be placed into the RXFIFO and an Rx Data Ready Interrupt will be
generated. The processor can then read the address byte and decide whether or not to
enable the receiver to accept the following data.
While the receiver is ENABLED (RS485CTRL bit 1 =’0’) all received bytes will be
accepted and stored in the RXFIFO regardless of whether they are data or address. When
an address character is received a parity error interrupt will be generated and the
processor can decide whether or not to disable the receiver.
RS-485 Auto Address Detection (AAD) mode
When both RS485CTRL register bits 0 (9-bit mode enable) and 2 (AAD mode enable) are
set, the UART is in auto address detect mode.
In this mode, the receiver will compare any address byte received (parity = ‘1’) to the 8-bit
value programmed into the RS485ADRMATCH register.
If the receiver is DISABLED (RS485CTRL bit 1 = ‘1’) any received byte will be discarded if
it is either a data byte OR an address byte which fails to match the RS485ADRMATCH
value.
When a matching address character is detected it will be pushed onto the RXFIFO along
with the partiy bit, and the receiver will be automatically enabled (RS485CTRL bit 1 will be
cleared by hardware). The receiver will also generate n Rx Data Ready Interrupt.
Table 248. UART0 RS485 Address Match register (U0/1RS485ADRMATCH -
0xE004 50450/0xE004 6050) bit description
Bit
Symbol
Description
Reset value
7:0
ADRMATCH
Contains the address match value.
0x00
Table 249. UART1 RS-485 Delay value register (U0/1RS485DLY - 0xE004 50454/0xE004 6054)
bit description
Bit
Symbol
Description
Reset value
7:0
DLY
Contains the direction control (RTS or DTR) delay
value. This register works in conjunction with an 8-bit
counter. <tbd>
0x00