82C50A_06 Datasheet, PDF (12/25 Pages) Intersil Corporation – CMOS Asynchronous

82C50A
data in the RBR before complete reception of the next
character result in the loss of the data in the Receiver
Register. The OE flag in the LSR register indicates the
overrun condition.
RBR Bits 0 thru 7
RBR (0)
RBR (1)
RBR (2)
RBR (3)
RBR (4)
RBR (5)
RBR (6)
RBR (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
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
transmission. 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
programmer 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
conditions 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
Identification 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 serviced 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
pending. When IIR(0) is low, an interrupt is pending, and the
lIR contents may be used as a pointer to the appropriate
interrupt 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
INTERRUPT IDENTIFICATION
INTERRUPT SET AND RESET FUNCTIONS
BIT 2 BIT 1 BIT 0
PRIORITY
LEVEL
INTERRUPT FLAG
INTERRUPT SOURCE
INTERRUPT
RESET CONTROL
X
X
1
None
None
1
1
0
First
Receiver Line Status
OE, PE, FE, or BI
LSR Read
1
0
0
Second
Received Data Available Receiver Data Available
RBR Read
0
1
0
Third
THRE
THRE
IIR Read if THRE is the
Interrupt Source or THR Write
0
0
0
Fourth
Modem Status
CTS, DSR, RI, DCD
MSR Read
NOTE: X = Not Defined, May Be 0 or 1
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FN2958.5
August 24, 2006