ST16C550 Exar Corporation, ST16C550 Datasheet - Page 11

• Manufacturer Part Number: ST16C550
• Description: UART WITH 16-BYTE FIFO
• Manufacturer: Exar Corporation

DMA Operation
The ST16C550 FIFO trigger level provides additional
flexibility to the user for block mode operation. The user
can optionally operate the transmit and receive FIFO’s
in the DMA mode (FCR bit-3). The DMA mode affects
the state of the -RXRDY and -TXRDY output pins. The
following tables show this:
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. 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 associ-
ated 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 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 serial-
izes 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,
-RXRDY pin:
Non-DMA mode
1 = FIFO empty
0 = at least 1 byte
-TXRDY pin:
Non-DMA mode
1 = at least 1 byte
0 = FIFO empty
Rev. 4.30
in FIFO
in FIFO
DMA mode
0 to 1 transition when FIFO
empties
1 to 0 transition when FIFO
reaches trigger level, or
timeout occurs
DMA mode
1 = FIFO is full
0 = FIFO has at least 1
empty location
11
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. The interrupts are still controlled by the
IER.
Figure 3, TYPICAL EXTERNAL CRYSTAL OSCILLA-
TOR CONNECTION
X T A L 1
2 2 -4 7 p F
C 1
Y 1
R 2
ST16C550
1 M
X T A L 2
1 .8 4 3 2 - 2 4 M H z
2 2 -4 7 p F
(O p tio n a l)
C 2
R 1
0 -1 2 0