The following section describes the operation of
the registers.
Bit 0
This bit enables the Received Data Available
Interrupt (and timeout interrupts in the FIFO mode)
when set to logic "1".
RECEIVE BUFFER REGISTER (RB)
Address Offset = 0H, DLAB = 0, READ ONLY
Bit 1
This bit enables the Transmitter Holding Register
Empty Interrupt when set to logic "1".
Bit 2
This bit enables the Received Line Status Interrupt
when set to logic "1". The error sources causing
the interrupt are Overrun, Parity, Framing and
Break. The Line Status Register must be read to
determine the source.
This register holds the received incoming data
byte. Bit 0 is the least significant bit, which is
transmitted and received first. Received data is
double buffered; this uses an additional shift
register to receive the serial data stream and
convert it to a parallel 8 bit word which is
transferred to the Receive Buffer register. The
shift register is not accessible.
Bit 3
This bit enables the MODEM Status Interrupt
when set to logic "1". This is caused when one of
the Modem Status Register bits changes state.
Bits 4 through 7
TRANSMIT BUFFER REGISTER (TB)
Address Offset = 0H, DLAB = 0, WRITE ONLY
This register contains the data byte to be
These bits are always logic "0".
transmitted.
The transmit buffer is double
buffered, utilizing an additional shift register (not
accessible) to convert the 8 bit data word to a
serial format. This shift register is loaded from the
Transmit Buffer when the transmission of the
previous byte is complete.
FIFO CONTROL REGISTER (FCR)
Address Offset = 2H, DLAB = X, WRITE
This is a write only register at the same location as
the IIR. This register is used to enable and clear
the FIFOs, set the RCVR FIFO trigger level. Note:
DMA is not supported. The UART1 and UART2
FCR’s are shadowed in the UART1 FIFO Control
Shadow Register (LD8:CRC3[7:0]) and UART2
FIFO Control Shadow Register (LD8:CRC4[7:0]).
INTERRUPT ENABLE REGISTER (IER)
Address Offset = 1H, DLAB = 0, READ/WRITE
The lower four bits of this register control the
enables of the five interrupt sources of the Serial
Port interrupt. It is possible to totally disable the
interrupt system by resetting bits 0 through 3 of
this register. Similarly, setting the appropriate bits
of this register to a high, selected interrupts can be
enabled. Disabling the interrupt system inhibits
the Interrupt Identification Register and disables
any Serial Port interrupt out of the chip. All other
system functions operate in their normal manner,
including the Line Status and MODEM Status
Registers. The contents of the Interrupt Enable
Register are described below.
Bit 0
Setting this bit to a logic "1" enables both the XMIT
and RCVR FIFOs. Clearing this bit to a logic "0"
disables both the XMIT and RCVR FIFOs and
clears all bytes from both FIFOs. When changing
from FIFO Mode to non-FIFO (16450) mode, data
is automatically cleared from the FIFOs. This bit
must be a 1 when other bits in this register are
written to or they will not be properly programmed.
73
SMSC [ SMSC CORPORATION ]
