ST16C550

13

Rev. 5.01

REGISTER FUNCTIONAL DESCRIPTIONS

The following table delineates the assigned bit functions for the twelve ST16C550 internal registers. The assigned

bit functions are more fully defined in the following paragraphs.

Table 4, ST16C550 INTERNAL REGISTERS

A2 A1 A0

Register

BIT-7

BIT-6

BIT-5

BIT-4

BIT-3

BIT-2

BIT-1

BIT-0

[Default]

Note *2

General Register Set

0

0

0

RHR [XX]

bit-7

bit-6

bit-5

bit-4

bit-3

bit-2

bit-1

bit-0

0

0

0

THR [XX]

bit-7

bit-6

bit-5

bit-4

bit-3

bit-2

bit-1

bit-0

0

0

1

IER [00]

0000

modem

receive

transmit

receive

status

line

holding

holding

interrupt

status

register

register

interrupt

0

1

0

FCR [00]

RCVR

RCVR

0

0

DMA

XMIT

RCVR

FIFO

trigger

trigger

mode

FIFO

FIFO

enable

(MSB)

(LSB)

select

reset

reset

0

1

0

ISR [01]

FIFO’s

FIFO’s

0

0

INT

INT

INT

INT

enabled

enabled

priority

priority

priority

status

bit-2

bit-1

bit-0

0

1

1

LCR [00]

divisor

set

set

even

parity

stop

word

word

latch

break

parity

parity

enable

bits

length

length

enable

bit-1

bit-0

1

0

0

MCR [00]

0

0

0

loopback

-OP2

-OP1

-RTS

-DTR

enable

1

0

1

LSR [60]

FIFO

trans.

trans.

break

framing

parity

overrun

receive

data

empty

holding

interrupt

error

error

error

data

error

empty

ready

1

1

0

MSR [X0]

CD

RI

DSR

CTS

delta

delta

delta

delta

-CD

-RI

-DSR

-CTS

1

1

1

SPR [FF]

bit-7

bit-6

bit-5

bit-4

bit-3

bit-2

bit-1

bit-0

Baud Rate Generator Divisor Registers. Accessible when LCR bit-7 is set to logic 1. Note 1*

0

0

0

DLL [XX]

bit-7

bit-6

bit-5

bit-4

bit-3

bit-2

bit-1

bit-0

0

0

1

DLM [XX]

bit-15

bit-14

bit-13

bit-12

bit-11

bit-10

bit-9

bit-8

Note *1: The BRG registers are accessible only when LCR bit-7 is set to a logic 1.

Note *2: The value represents the register’s initialized HEX value. An “X” signifies a 4-bit un-initialized nibble.