Bit 1
Address Offset =7H, DLAB =X, READ/WRITE
This 8 bit read/write register has no effect on the
operation of the Serial Port. It is intended as a
scratchpad register to be used by the programmer
to hold data temporarily.
Delta Data Set Ready (DDSR). Bit 1 indicates that
the nDSR input has changed state since the last
time the MSR was read.
Bit 2
Trailing Edge of Ring Indicator (TERI). Bit 2
indicates that the nRI input has changed from logic
"0" to logic "1".
PROGRAMMABLE BAUD RATE GENERATOR
(AND DIVISOR LATCHES DLH, DLL)
Bit 3
The Serial Port contains a programmable Baud
Rate Generator that is capable of taking any clock
input (DC to 3 MHz) and dividing it by any divisor
from 1 to 65535. This output frequency of the
Baud Rate Generator is 16x the Baud rate. Two 8
bit latches store the divisor in 16 bit binary format.
These Divisor Latches must be loaded during
initialization in order to insure desired operation of
the Baud Rate Generator. Upon loading either of
the Divisor Latches, a 16 bit Baud counter is
immediately loaded. This prevents long counts on
initial load. If a 0 is loaded into the BRG registers
the output divides the clock by the number 3. If a
1 is loaded the output is the inverse of the input
oscillator. If a two is loaded the output is a divide
by 2 signal with a 50% duty cycle. If a 3 or greater
is loaded the output is low for 2 bits and high for
the remainder of the count. The input clock to the
BRG is a 1.8462 MHz clock.
Delta Data Carrier Detect (DDCD). Bit 3 indicates
that the nDCD input to the chip has changed state.
Note: Whenever bit 0, 1, 2, or 3 is set to a logic
"1", a MODEM Status Interrupt is generated.
Bit 4
This bit is the complement of the Clear To Send
(nCTS) input. If bit 4 of the MCR is set to logic "1",
this bit is equivalent to nRTS in the MCR.
Bit 5
This bit is the complement of the Data Set Ready
(nDSR) input. If bit 4 of the MCR is set to logic
"1", this bit is equivalent to DTR in the MCR.
Bit 6
This bit is the complement of the Ring Indicator
(nRI) input. If bit 4 of the MCR is set to logic "1",
this bit is equivalent to OUT1 in the MCR.
Bit 7
This bit is the complement of the Data Carrier
Detect (nDCD) input. If bit 4 of the MCR is set to
logic "1", this bit is equivalent to OUT2 in the MCR.
SCRATCHPAD REGISTER (SCR)
Table 35 shows the baud rates possible with a
1.8462 MHz crystal.
79
SMSC [ SMSC CORPORATION ]
