CIRCC datasheet(40/70 Pages) SMSC

40 / 70 page

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

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 SMSC CIrCC.

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 through 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.

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.

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