XR16C2850
The 2850 provides RS-485 half dulpex control signal
to select the external transceiver direction. Auto RS-
485 control pin (-RTS) is not activated after reset. To
activate the direction control function, user has to set
EFR Bit-4, and FCTR Bit-3 to 1. The -RTS pin is
normally high for receive mode and it will go low when
transmitter starts transmitting data.
GENERAL DESCRIPTION
The XR16C2850 provides serial asynchronous re-
ceive data synchronization, parallel-to-serial and se-
rial-to-parallel data conversions for both the transmit-
ter and receiver sections. These functions are neces-
sary for converting the serial data stream into parallel
data that is required with digital data systems. Syn-
chronization for the serial data stream is accom-
plished by adding start and stops bits to the transmit
data to form a data character (character orientated
protocol). Dataintegrityisinsuredbyattachingaparity
bit to the data character. The parity bit is checked by
the receiver for any transmission bit errors. The elec-
tronic circuitry to provide all these functions is fairly
complex especially when manufactured on a single
integrated silicon chip. The XR16C2850 represents
such an integration with greatly enhanced features.
The 2850 is fabricated with an advanced CMOS
process.
FUNCTIONAL DESCRIPTIONS
UART A-B Functions
The UART provides the user with the capability to Bi-
directionally transfer information between an external
CPU and an external serial communication device. A
logic 0 on chip select pins -CSA and/or -CSB allows
the user to configure, send data, and/or receive data
via UART channels A-B. Individual channel select
functions are shown in Table 2 below.
Table 2, SERIAL PORT SELECTION GUIDE
The 2850 is an upward solution that provides 128
bytes of transmit and receive FIFO memory, instead
of 16 bytes provided in the 16C2550. The 2850 is
designedtoworkwithhighspeedmodemsandshared
network environments, that require fast data process-
ing time. Increased performance is realized in the
2850 by the larger transmit and receive FIFO. This
allowstheexternalprocessortohandlemorenetwork-
ing tasks within a given time. For example, the
ST16C2550 with a 16 byte FIFO, unloads 16 bytes of
receivedatain1.53ms(thisexampleusesacharacter
length of 11 bits, including start/stop bits at
115.2Kbps). This means the external CPU will have to
service the receive FIFO at 1.53 ms intervals. How-
ever with the 128 byte FIFO in the 2850, the data
buffer will not require unloading/loading for 12.2 ms.
This increases the service interval giving the external
CPU additional time for other applications and reduc-
ing the overall UART interrupt servicing time. In
addition, the programmable levels of FIFO trigger
interrupt and automatic hardware/software flow con-
trol is uniquely provided for maximum data throughput
performance especially when operating in a multi-
channel environment. The combination of the above
greatly reduces the bandwidth requirement of the
externalcontrollingCPU, increasesperformance, and
reduces power consumption.
CHIP SELECT
Function
-CS A-B = 1s
-CS A = 0
-CS B = 0
None
UART CHANNEL A
UART CHANNEL B
Internal Registers
The 2850 provides 15 internal registers for monitoring
and control. These resisters are shown in Table 3.
Twelve registers are similar to those already available
in the standard 16C2550. These registers function as
data holding registers (THR/RHR), interrupt status
and control registers (IER/ISR), a FIFO control regis-
ter (FCR), line status and control registers, (LCR/
LSR), modem status and control registers (MCR/
MSR), programmable data rate (clock) control regis-
ters (DLL/DLM), and a user accessible scratchpad
register (SPR). Beyond the general 16C2550 features
and capabilities, the 2850 offers an enhanced feature
register set (EFR, Xon/Xoff 1-2) that provides on
board hardware/software flow control. Register func-
tions are more fully described in the following para-
graphs.
Rev. 1.00P
8
EXAR [ EXAR CORPORATION ]
