ST16C454(1994) 查看數據表（PDF） - Exar Corporation

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ST16C454
(Rev.:1994)

QUAD UNIVERSAL ASYNCHRONOUS RECEIVER/TRANSMITTER (UART)

Exar Corporation 

ST16C454/68C454

Figure 8, Crystal oscillator connection

X1

1.8432 MHz

C1

C2

22pF

33pF

Loop-back 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. MCR register bits 0-3 are used

for controlling loop-back diagnostic testing. In the

loop-back mode OP1 and OP2 in the MCR register

(bits 3/2) 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 asso-

ciated interface pins, and instead are connected to-

gether internally (See Figure 12). The -CTS, -DSR, -

CD, and -RI are disconnected from their normal

modem control inputs pins, and instead are connected

internally 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 connec-

tion. 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 com-

pares 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 con-

trolled by the IER.

Rev. 3.20

12

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