The following section describes the operation of the
registers.
Bit 1
This bit enables the Transmitter Holding Register Empty
Interrupt when set to logic "1".
RECEIVE BUFFER REGISTER (RB)
Address Offset = 0H, DLAB = 0, READ ONLY
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.
TRANSMIT BUFFER REGISTER (TB)
Address Offset = 0H, DLAB = 0, WRITE ONLY
Bits 4 through 7
These bits are always logic "0".
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.
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.
INTERRUPT ENABLE REGISTER (IER)
Address Offset = 1H, DLAB = 0, READ/WRITE
Bit 0
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 FDC37C669. 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.
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.
Bit 1
Setting this bit to a logic "1" clears all bytes in the RCVR
FIFO and resets its counter logic to 0. The shift register
is not cleared. This bit is self-clearing.
Bit 0
This bit enables the Received Data Available Interrupt
(and timeout interrupts in the FIFO mode) when set to
logic "1".
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