scn68681 NXP Semiconductors, scn68681 Datasheet - Page 8

• Manufacturer Part Number: scn68681
• Description: Dual Asynchronous Receiver/transmitter Duart
• Manufacturer: NXP Semiconductors

communications Channels A and B, input port and output port. Refer
allow read and write operations to take place between the controlling
determine all currently active interrupting conditions. When IACKN is
X1/CLK. The clock serves as the basic timing reference for the Baud
Philips Semiconductors
BLOCK DIAGRAM
The SCN68681 DUART consists of the following eight major
sections: data bus buffer, operation control, interrupt control, timing,
to the Block Diagram.
Data Bus Buffer
The data bus buffer provides the interface between the external and
internal data buses. It is controlled by the operation control block to
CPU and the DUART.
Operation Control
The operation control logic receives operation commands from the
CPU and generates appropriate signals to internal sections to
control device operation. It contains address decoding and read and
write circuits to permit communications with the microprocessor via
the data bus buffer. The DTACKN output is asserted during write
and read cycles to indicate to the CPU that data has been latched
on a write cycle, or that valid data is present on the bus on a read
cycle.
Interrupt Control
A single active-LOW interrupt output (INTRN) is provided which is
activated upon the occurrence of any of eight internal events.
Associated with the interrupt system are the Interrupt Mask Register
(IMR) and the Interrupt Status Register (ISR), the Auditory Control
Register(ACR) and the Interrupt Vector Register (IVR). The IMR
may be programmed to select only certain conditions to cause
INTRN to be asserted. The ISR can be read by the CPU to
asserted, and the DUART has an interrupt pending, the DUART
responds by placing the contents of the IVR register on the data bus
and asserting DTACKN.
Outputs OP3-OP7 can be programmed to provide discrete interrupt
outputs for the transmitter, receivers, and counter/timer.
Timing Circuits
The timing block consists of a crystal oscillator, a baud rate
generator, a programmable 16-bit counter/timer, and four clock
selectors. The crystal oscillator operates directly from a crystal
connected across the X1/CLK and X2 inputs. If an external clock of
the appropriate frequency is available, it may be connected to
Rate Generator (BRG), the counter/timer, and other internal circuits.
A clock signal within the limits specified in the specifications section
of this data sheet must always be supplied to the DUART. If an
external is used instead of a crystal, X1 should be driven using a
configuration similar to the one in Figure 9.
The baud rate generator operates from the oscillator or external
clock input and is capable of generating 18 commonly used data
communications baud rates ranging from 50 to 38.4 k baud. The
clock outputs from the BRG are at 16X the actual baud rate. The
counter/timer can be used as a timer to produce a 16X clock for any
other baud rate by counting down the crystal clock or an external
clock. The four clock selectors allow the independent selection, for
each receiver and transmitter, of any of these baud rates or external
timing signal.
Counter/Timer (C/T)
The counter timer is a 16 bit programmable divider that operates
one of three modes: Counter, Timer or Time Out mode. In all three
modes it uses the 16-bit value loaded to the CTUR and CTLR
registers. (Counter timer upper and lower preset registers).
2004 Mar 02
Dual asynchronous receiver/transmitter (DUART)
8
performing a read operation at address H‘D’. A HIGH input results in
The counter operates as a down counter and sets its output bit in
the ISR (Interrupt Status Register) each time it passes through 0.
The output of the counter/timer may be seen on one of the OP pins
or as an Rx or Tx clock.
The Timer/Counter is controlled with six (6) “commands”; Start C/T,
Stop C/T, write C/T, preset registers, read C/T value, set or reset
time out mode.
Please see the detail of the commands under the Counter/Timer
register descriptions.
Communications Channels A and B
Each communications channel of the SCN68681 comprises a
full-duplex asynchronous receiver/transmitter (DUART). The
operating frequency for each receiver and transmitter can be
selected independently from the baud rate generator, the counter
timer, or from an external input.
The transmitter accepts parallel data from the CPU, converts it to a
serial bit stream, inserts the appropriate start, stop, and optional
parity bits and outputs a composite serial stream of data on the TxD
output pin. The receiver accepts serial data on the RxD pin,
converts this serial input to parallel format, checks for start bit, stop
bit, parity bit (if any), or break condition and sends an assembled
character to the CPU.
The input port pulse detection circuitry uses a 38.4 kHz sampling
clock derived from one of the baud rate generator taps. This results
in a sampling period of slightly more than 25 s (assuming that the
clock input is 3.6864 MHz). The detection circuitry, in order to
guarantee a true change in level has occurred, requires that two
successive samples at the new logic level be observed. As a
consequence, the minimum duration of the signal change is 25 s if
the transition occurs coincident with the first sample pulse. The
50 s time refers to the situation in which the change of state is just
missed and the first change of state is not detected until 25 s later.
Input Port
The inputs to this unlatched 6-bit port can be read by the CPU by
a logic 1 while a LOW input results in a logic 0. D7 will always read
as a logic 1 and D6 will reflect the level of IACKN. The pins of this
port can also serve as auxiliary inputs to certain portions of the
DUART logic.
Four change-of-state detectors are provided which are associated
with inputs IP3, IP2, IP1 and IP0. A HIGH-to-LOW or LOW-to-HIGH
transition of these inputs, lasting longer than 25 to 50 s, will set the
corresponding bit in the input port change register. The bits are
cleared when the register is read by the CPU. Any change-of-state
can also be programmed to generate an interrupt to the CPU.
All the IP pins have a small pull-up device that will source 1 to 4 A
of current from V
V
Output Port
The 8-bit multipurpose output port can be used as a general
purpose output port, in which case the outputs are the complements
CC
In the timer mode it generates a square wave.
In the counter mode it generates a time delay.
In the time out mode it monitors the receiver data flow and signals
data flow has paused. In the time out mode the receiver controls
the starting/stopping of the C/T.
connections if they are not used.
CC
. These pins do not require pull-up devices or
SCN68681
Product data