82C50A_06 Datasheet, PDF (7/25 Pages) Intersil Corporation – CMOS Asynchronous

82C50A
Accessible Registers
The three types of internal registers in the 82C50A used in
the operation of the device are control, status, and data
registers. The control registers are the Bit Rate Select
Register DLL and DLM, Line Control Register, Interrupt
Enable Register and the Modem Control registers, while the
status registers are the Line Status Registers and the
Modem Status Register. The data registers are the Receiver
Buffer Register and Transmitter Holding Register. The
Address, Read, and Write inputs are used in conjunction
with the Divisor Latch Access Bit in the Line Control Register
(LCR(7)) to select the register to be written or read (see
Table 1.). Individual bits within these registers are referred to
by the register mnemonic and the bit number in parenthesis.
An example, LCR(7) refers to Line Control Register Bit 7.
The Transmitter Buffer Register and Receiver Buffer
Register are data registers holding from 5-8 data bits. If less
than eight data bits are transmitted, data is right justified to
the LSB. Bit 0 of a data word is always the first serial data bit
received and transmitted. The 82C50A data registers are
double buffered so that read and write operations can be
performed at the same time the UART is performing the
parallel to serial and serial to parallel conversion. This
provides the microprocessor with increased flexibility in its
read and write timing.
TABLE 1. ACCESSING 82C50A INTERNAL REGISTERS
DLAB A2 A1 A0 MNEMONIC
REGISTER
0
000
RBR
Receiver Buffer
Register (read only)
0
000
THR
Transmitter Holding
Register (write only)
0
001
lER
Interrupt Enable
Register
X
010
IIR
Interrupt Identification
Register
(read only)
X
011
LCR
Line Control Register
X
100
MCR
Modem Control
Register
X
101
LSR
Line Status Register
X
110
MSR
Modem Status
Register
X
111
SCR Scratch Register
1
000
DLL
Divisor Latch (LSB)
1
001
DLM Divisor Latch (MSB)
NOTE: X = “Don’t Care”, 0 = Logic Low, 1 = Logic High
Line Control Register (LCR)
LCR LCR LCR LCR LCR LCR LCR LCR
7
6
5
4
3
2
1
0
Word
Length
Select
0 0 = 5 Data Bits
0 1 = 6 Data Bits
1 0 = 7 Data Bits
1 1 = 8 Data Bits
Stop
Bit
Select
0 = 1 Stop Bit
1 = 1.5 Stop Bits if 5 Data Bit Word Length is Selected 2 Stop Bits if 6,
7, or 8 Data Bit Word Length is Selected
Parity
Enable
0 = Parity Disabled
1 = Parity Enabled (Generated & Checked)
Even Parity 0 = Odd Parity When Parity is Enabled
Select
1 = Even Parity When Parity is Enabled
Stick Parity
0 = Stick Parity Disabled
1 = When Parity is Enabled Forces the Transmission and Checking of
a Parity Bit of a Known State. Parity Bit Forced to a Logic 1 if LCR
(4) = 0 or to a Logic 0 If LCR (4) = 1.
Break
Control
0 = Break Disabled
1 = Break Enabled. The Serial Output (SOUT) is Forced to the Spacing
(Logic 0) State.
Divisor Latch 0 = Must be Low to Access the Receiver Buffer. Transmitter Holding
Access Bit
Register or the Interrupt Enable Register.
1 = Must be High to Access the Divisor Latches DLL and DLM of the
Baud Rate Generator During a Read or Write Operation.
7
FN2958.5
August 24, 2006