XR68C92_05 Datasheet, PDF (7/33 Pages) Exar Corporation – DUAL UNIVERSAL ASYNCHRONOUS RECEIVER AND TRANSMITTER

XR68C92/192
INTERNAL CONTROL LOGIC
The internal control logic of the XR68C92/192 receives
operation commands from the central processing unit
(CPU) and generates appropriate signals to the inter-
nal sections to control device operation. The internal
control logic takes in the following inputs:
• -CS, which is the XR68C92/192 chip-select;
• R/-W which allows data transfers between the CPU
and XR68C92/192via the data bus (D0 to D7);
• four register-select lines (A0 through A3) which are
decoded to allow access to the registers within the
XR68C92/192;
• -RESET (reset), which initializes or resets all
outputs and internal registers.
COMMUNICATION CHANNELS A AND B
Each communication channel includes a full-duplex
asynchronous receiver/transmitter (UART). The oper-
ating frequency for each receiver and each transmitter
can be selected independently from the baud rate
generator, the Counter/Timer (C/T), or from an exter-
nal clock. The transmitter accepts parallel data from
the CPU, converts it to a serial bit stream in the form of
a character and outputs it on the Transmit Data output
pin (TXA, TXB). The character consists of start, stop,
and optional parity bits, The receiver accepts serial
data on the Receive Data input pin (RXA, RXB),
converts this serial input to parallel format, checks for
a start bit, stop bit, parity bit (if any), framing error,
overrun or break condition, and transfers the data byte
to the CPU during read operations.
TIMING LOGIC
The timing logic consists of
• a crystal oscillator,
• a baud rate generator (BRG),
• clock selector logic, and
• a programmable 16-bit counter/timer (C/T).
The crystal oscillator operates directly from a typical
3.6864 MHz crystal connected across the XTAL1 and
XTAL2 inputs or from an external clock of the appropri-
ate frequency connected to XTAL1. The XTAL1 clock
serves as the basic timing reference for the baud rate
generator, the C/T, and other internal circuits.
The baud rate generator operates from the XTAL1
clock input and can generate 28 commonly used data
communication baud rates (if a typical 3.6864MHz
crystal or clock is used) ranging from 50 to 230.4kbps
by producing internal clock outputs at 16 times the
actual baud rate. In addition, other baud rates can be
derived by connecting 16X or 1X clocks to multi-
purpose input port pins IP3 - IP6 that have alternate
functions as receiver or transmitter clock inputs.
Clock selector logic consists of the clock selector
register (CSRA, CSRB), bits 0 & 2 of Mode Register 0
(MR0A, MR0B) and bit-7 of Auxilliary Control Register
(ACR). These allow various combinations of these
baud rates for receiver and transmitter of each chan-
nel. See Baud Rate Table on page 18 for more details.
The programmable 16-bit counter/timer (C/T) can pro-
duce a 16X clock for other baud rates by counting down
its programmed clock source. Users can program the
16 bit C/T within the XR68C92/192 to use one of
several clock sources as its input. The output of the C/
T is available to the internal clock selectors and can
also be programmed to appear at output OP3. In the
timer mode, the C/T acts as a programmable divider
and can generate a square-wave output at OP3. In the
counter mode, the C/T can be started and stopped
under program control. When stopped, the CPU can
read its contents. The counter counts down the num-
ber of pulses stored in the concatenation of the C/T
upper register and C/T lower register and produces an
interrupt. This is a system-oriented feature that can be
used to record timeouts when implementing various
application protocols.
INTERRUPT CONTROL LOGIC
The following registers are associated with the inter-
rupt control logic:
• Interrupt Mask Register (IMR)
• Interrupt Status Register (ISR)
• Auxiliary Control Register (ACR)
• Interrupt Vector Register (IVR)
A single active-low interrupt output (-INT) can notify the
CPU that any of eight internal events has occurred.
These eight events are described in the discussion of
the interrupt status register (ISR). User can program
the interrupt mask register (IMR) to allow only certain
conditions to cause -INT to be asserted while the CPU
can read the ISR to determine all currently active
interrupting conditions. When an active-low interrupt
acknowledge signal (-IACK) from the CPU is asserted
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