XR68C92 datasheet(7/33 Pages) EXAR

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XR68C92/192

Rev. 1.33

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

(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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