SC16IS741A All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
Product data sheet Rev. 1 — 18 March 2013 30 of 55
NXP Semiconductors
SC16IS741A
Single UART with I
2
C-bus/SPI interface, 64-byte FIFOs, IrDA SIR
9. RS-485 features
9.1 Auto RS-485 RTS control
Normally the RTS pin is controlled by MCR bit 1, or if hardware flow control is enabled, the
logic state of the RTS
pin is controlled by the hardware flow control circuitry. EFCR
register bit 4 will take the precedence over the other two modes; once this bit is set, the
transmitter will control the state of the RTS
pin. The transmitter automatically asserts the
RTS
pin (logic 0) once the host writes data to the transmit FIFO, and de-asserts RTS pin
(logic 1) once the last bit of the data has been transmitted.
To use the auto RS-485 RTS
mode the software would have to disable the hardware flow
control function.
9.2 RS-485 RTS output inversion
EFCR bit 5 reverses the polarity of the RTS pin if the UART is in auto RS-485 RTS mode.
When the transmitter has data to be sent it de-asserts the RTS
pin (logic 1), and when the
last bit of the data has been sent out the transmitter asserts the RTS
pin (logic 0).
9.3 Auto RS-485
EFCR bit 0 is used to enable the RS-485 mode (multidrop or 9-bit mode). In this mode of
operation, a ‘master’ station transmits an address character followed by data characters
for the addressed ‘slave’ stations. The slave stations examine the received data and
interrupt the controller if the received character is an address character (parity bit = 1).
To use the auto RS-485 mode the software would have to disable the hardware and
software flow control functions.
9.3.1 Normal multidrop mode
The 9-bit Mode in EFCR (bit 0) is enabled, but not Special Character Detect (EFR bit 5).
The receiver is set to Force Parity 0 (LCR[5:3] = 111) in order to detect address bytes.
With the receiver initially disabled, it ignores all the data bytes (parity bit = 0) until an
address byte is received (parity bit = 1). This address byte will cause the UART to set the
parity error. The UART will generate a line status interrupt (IER bit 2 must be set to ‘1’ at
this time), and at the same time puts this address byte in the RX FIFO. After the controller
examines the byte it must make a decision whether or not to enable the receiver; it should
enable the receiver if the address byte addresses its ID address, and must not enable the
receiver if the address byte does not address its ID address.
If the controller enables the receiver, the receiver will receive the subsequent data until
being disabled by the controller after the controller has received a complete message
from the ‘master’ station. If the controller does not disable the receiver after receiving a
message from the ‘master’ station, the receiver will generate a parity error upon receiving
another address byte. The controller then determines if the address byte addresses its ID
address, if it is not, the controller then can disable the receiver. If the address byte
addresses the ‘slave’ ID address, the controller takes no further action, the receiver will
receive the subsequent data.
