11
FN2958.6
August 19, 2015
82C50A
MSR(3) Delta Data Carrier Detect (DDCD): DDCD
indicates that the DCD
input (Pin-36) to the 82C50A has
changed state since the last time it was read by the CPU.
MSR(4) Clear to Send (CTS): Clear to Send (CTS) is the
status of the CTS
input (Pin-36) from the modem indicating
to the 82C50A that the modem is ready to receive data from
the 62C50A transmitter output (SOUT). If the 82C50A is in
the loop mode (MCR(4)=1), MSR(4) is equivalent to RTS in
the MCR.
MSR(5) Data Set Ready (DSR): Data Set Ready (DSR) is a
status of the DSR
input (Pin-37) from the modem to the
82C50A which indicates that the modem is ready to provide
received data to the 82C50A receiver circuitry. If the 82C50A
is in the loop mode (MCR(4) = 1), MSR(5) is equivalent to
DTR in the MCR.
MSR(6) Ring Indicator MSR(6): Indicates the status of the
RI input (Pin-39). If the 82C50A is in the loop mode (MCR(4)
= 1), MSR(6) is equivalent to OUT1 in the MCR.
MSR(7) Data Carrier Detect (MSR(7)): Data Carrier Detect
indicates the status of the Data Carrier Detect
(DCD) input
(Pin-38). If the 82C50A is in the loop mode (MCR(4) = 1),
MSR(4) is equivalent to OUT2 of the MCR.
The modem status inputs (RI, DCD, DSR and CTS) reflect
the modem input lines with any change of status. Reading
the MSR register will clear the delta modem status
indications but has no effect on the status bits. The
status bits reflect the state of the input pins regardless of the
mask control signals. If a DCTS, DDSR, TERI, or DDCD are
true and a state change occurs during a read operation
(DlSTR, DISTR
), the state change is not indicated in the
MSR. If DCTS, DDSR, TERI, or DDCD are false and a state
change occurs during a read operation, the state change is
indicated after the read operation.
For LSR and MSR, the setting of status bits is inhibited
during status register read (DISTR, DlSTR
) operations. If a
status condition is generated during a read (DlSTR, DISTR
)
operation, the status bit is not set until the trailing edge of the
read (DISTR, DISTR
).
If a status bit is set during a read (DlSTR, DISTR) operation,
and the same status condition occurs, that status bit will be
cleared at the trailing edge of the read (DlSTR, DISTR
)
instead of being set again.
BAUD RATE SELECT REGISTER (BRSR)
The 82C50A contains a programmable Baud Rate
Generator (BRG) that divides the clock (DC to 10MHz) by
any divisor from 1 to 2
16
-1 (see also BRG description). The
output frequency of the Baud Generator is 16X the data rate
[divisor # = frequency input (baud rate x 16)]. Two 8-bit
divisor latch registers store the divisor in a 16-bit binary
format. These Divisor Latch registers must be loaded during
initialization. Upon loading either of the Divisor Latches, a
16-bit Baud counter is immediately loaded. This prevents
long counts on initial load.
Sample Divisor Number Calculation:
Given: Desired Baud Rate 1200 Baud
Frequency Input 1.8432MHz
Formula: Divisor # = Frequency Input (Baud Rate x 16)
Divisor # = 1843200 (1200 x 16)
Answer: Divisor # = 96 = 60
HEX
DLL = 01100000
DLM = 00000000
Check: The Divisor # 96 will divide the input frequency
1.8432MHz down to 19200 which is 16 times the
desired baud rate.
RECEIVER BUFFER REGISTER (RBR)
The receiver circuitry in the 82C50A is programmable for 5,
6, 7 or 8 data bits per character. For words of less than 8
bits, the data is right justified to the least significant bit (LSB
= Data Bit 0 (RBR(0)). Data Bit 0 of a data word (RBR(0)) is
the first data bit received. The unused bits in a character less
than 8 bits are output low to the parallel output by the
82C50A.
Received data at the SIN input pin is shifted into the
Receiver Shift Register by the 16X clock provided at the
RCLK input. This clock is synchronized to the incoming data
based on the position of the start bit. When a complete
character is shifted into the Receiver Shift Register, the
assembled data bits are parallel loaded into the Receiver
Buffer Register. The DR flag in the LSR register is set.
Double buffering of the received data permits continuous
reception of data without losing received data. While the
Receiver Shift Register is shifting a new character into the
82C50A, the Receiver Buffer Register is holding a previously
received character for the CPU to read. Failure to read the
Divisor Latch Least Significant BYTE
DLL (0) Bit 0
DLL (1) Bit 1
DLL (2) Bit 2
DLL (3) Bit 3
DLL (4) Bit 4
DLL (5) Bit 5
DLL (6) Bit 6
DLL (7) Bit 7
Divisor Latch Most Significant BYTE
DLM (0) Bit 8
DLM (1) Bit 9
DLM (2) Bit 10
DLM (3) Bit 11
DLM (4) Bit 12
DLM (5) Bit 13
DLM (6) Bit 14
DLM (7) Bit 15
