16C450 查看數據表（PDF） - Exar Corporation

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16C450

UNIVERSAL ASYNCHRONOUS RECEIVER/TRANSMITTER (UART)

Exar Corporation 

ST16C450

Figure 3, EXTERNAL CRYSTAL OSCILLATOR

CONNECTION

R1

50-120

R2

1M

X1

1.8432 MHz

C1

C2

22pF

33pF

associated interface pins, and instead are connected

together internally (See Figure 4). The -CTS, -DSR, -CD,

and -RI are disconnected from their normal modem

control inputs pins, and instead are connected inter-

nally to -DTR, -RTS, -OP1 and -OP2. Loop-back test

data is entered into the transmit holding register via the

user data bus interface, D0-D7. The transmit UART

serializes the data and passes the serial data to the

receive UART via the internal loop-back connection. The

receive UART converts the serial data back into parallel

data that is then made available at the user data

interface, D0-D7. The user optionally compares the

received data to the initial transmitted data for verifying

error free operation of the UART TX/RX circuits.

In this mode , the receiver and transmitter interrupts are

fully operational. The Modem Control Interrupts are also

operational. However, the interrupts can only be read

using lower four bits of the Modem Control Register

(MCR bits 0-3) instead of the four Modem Status

Register bits 4-7. The interrupts are still controlled by

the IER.

Loopback Mode

The internal loop-back capability allows onboard diag-

nostics. In the loop-back mode the normal modem

interface pins are disconnected and reconfigured for

loop-back internally. In this mode MSR bits 4-7 are

also disconnected. However, MCR register bits 0-3

can be used for controlling loop-back diagnostic test-

ing. In the loop-back mode -OP1 and -OP2 in the MCR

register (bits 0-1) control the modem -RI and -CD

inputs respectively. MCR signals -DTR and -RTS (bits

0-1) are used to control the modem -CTS and -DSR

inputs respectively. The transmitter output (TX) and

the receiver input (RX) are disconnected from their

Rev. 4.20

10

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