
SMSC DS – FDC37N958FR
Page 81
Rev. 09/01/99
The following section describes the operation of
the registers.
RECEIVE BUFFER REGISTER (RB)
Address Offset = 0H, DLAB = 0, READ ONLY
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.
TRANSMIT BUFFER REGISTER (TB)
Address Offset = 0H, DLAB = 0, WRITE ONLY
This register contains the data byte to be
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.
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
FDC37N958FR. 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
This bit enables the Received Data Available
Interrupt (and timeout interrupts in the FIFO
mode) when set to logic "1".
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.
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 - 7
These bits are always logic "0".
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.
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.