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82C50A
(Rev.:1997)

CMOS Asynchronous Communications Element

Intersil 

82C50A

TRANSMITTER HOLDING REGISTER (THR)

The Transmitter Holding Register (THR) holds parallel data

from the data bus (D0-D7) until the Transmitter Shift Register

is empty and ready to accept a new character for transmis-

sion. The transmitter and receiver word length and number

of stop bits are the same. If the character is less than eight

bits, unused bits at the microprocessor data bus are ignored

by the transmitter.

Data Bit 0 (THR(0)) is the first serial data bit transmitted. The

THRE flag (LSR(5)) reflect the status of the THR. The TEMT

flag (LSR(6)) indicates if both the THR and TSR are empty.

THR Bits 0 thru 7

THR (0)

THR (1)

THR (2)

THR (3)

THR (4)

THR (5)

THR (6)

THR (7)

Data Bit 0

Data Bit 1

Data Bit 2

Data Bit 3

Data Bit 4

Data Bit 5

Data Bit 6

Data Bit 7

SCRATCHPAD REGISTER (SCR)

This 8-bit Read/Write register has no effect on the 82C50A.

It is intended as a scratchpad register to be used by the pro-

grammer to hold data temporarily.

SCR Bits 0 thru 7

SCR (0)

SCR (1)

SCR (2)

SCR (3)

SOR (4)

SCR (5)

SOR (6)

SCR (7)

Data Bit 0

Data Bit 1

Data Bit 2

Data Bit 3

Data Bit 4

Data Bit 5

Data Bit 6

Data Bit 7

Interrupt Structure

INTERRUPT IDENTIFICATION REGISTER (IIR)

The 82C50A has interrupt capability for interfacing to current

microprocessors. In order to minimize software overhead

during data character transfers, the 82C50A prioritizes

interrupts into four levels. The four levels of interrupt condi-

tions are as follows:

1. Receiver Line Status (Priority 1)

2. Received Data Ready (Priority 2)

3. Transmitter Holding Register Empty (Priority 3)

4. Modem Status (Priority 4).

Information indicating that a prioritized interrupt is pending

and the type of interrupt is stored in the Interrupt Identifica-

tion Register (IIR). When addressed during chip select time,

the lIR indicates the highest priority interrupt pending. No

other interrupts are acknowledged until the interrupt is ser-

viced by the CPU. The contents of the lIR are indicated in

Table 2 and are described below.

IIR(0): IIR(0) can be used in either a hardwired prioritized or

polled environment to indicate whether an interrupt is pend-

ing. When IIR(0) is low, an interrupt is pending, and the lIR

contents may be used as a pointer to the appropriate inter-

rupt service routine. When lIR(0) is high, no interrupt is

pending.

IlR(1) and IIR(2): llR(1) and IlR(2) are used to identify the

highest priority interrupt pending as indicated in Table 2.

lIR(3) - IIR(7): These five bits of the lIR are logic 0.

TABLE 2. INTERRUPT IDENTIFICATION REGISTER

BIT 2

X

1

1

0

INTERRUPT IDENTIFICATION

BIT 1

X

1

BIT 0

1

0

PRIORITY

LEVEL

First

0

0

Second

1

0

Third

0

0

0

NOTE: X = Not Defined, May Be 0 or 1

Fourth

INTERRUPT SET AND RESET FUNCTIONS

INTERRUPT

FLAG

INTERRUPT

SOURCE

INTERRUPT

RESET CONTROL

None

None

Receiver Line

Status

OE, PE, FE, or

BI

LSR Read

Received Data

Available

Receiver Data

Available

RBR Read

THRE

THRE

IIR Read if THRE is the

Interrupt Source or

THR Write

Modem Status

CTS, DSR, RI,

DCD

MSR Read

INTERRUPT ENABLE REGISTER (IER)

The Interrupt Enable Register (IER) is a Write register used

to independently enable the four 82C50A interrupts which

activate the interrupt (lNTRPT) output. All interrupts are dis-

abled by resetting IER(0) - IER(3) of the Interrupt Enable

Register. Interrupts are enabled by setting the appropriate

bits of the IER high. Disabling the interrupt system inhibits

the Interrupt Identification Register and the active (high)

INTRPT output. All other system functions operate in their

normal manner, including the setting of the Line Status and

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