SC16C850 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2010. All rights reserved.
Product data sheet Rev. 2 — 11 November 2010 19 of 55
NXP Semiconductors
SC16C850
2.5 to 3.3 V UART with 128-byte FIFOs and IrDA encoder/decoder
6.13 RS-485 features
6.13.1 Auto RS-485 RTS control
Normally the RTS pin is controlled by MCR[1], or if hardware flow control is enabled, the
logic state of the RTS
pin is controlled by the hardware flow control circuitry. AFCR2[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.
6.13.2 RS-485 RTS inversion
AFCR2[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 will 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).
6.13.3 Auto 9-bit mode (RS-485)
AFCR2[0] is used to enable the 9-bit mode (Multi-drop or RS-485 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 automatic 9-bit mode, the software would have to disable the hardware and
software flow control functions.
6.13.3.1 Normal Multi-drop mode
The 9-bit Mode in AFCR2[0] is enabled, but not Special Character Detect (EFR[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[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, and the receiver
will receive the subsequent data.
