Bit 0
to logic "1", this bit is equivalent to OUT2 in the MCR.
Delta Clear To Send (DCTS). Bit 0 indicates that the
nCTS input to the chip has changed state since the
last time the MSR was read.
SCRATCHPAD REGISTER (SCR)
Address Offset =7H, DLAB =X, READ/WRITE
Bit 1
This 8 bit read/write register has no effect on the
Delta Data Set Ready (DDSR). Bit 1 indicates that the
nDSR input has changed state since the last time the
MSR was read.
operation of the Serial Port.
scratchpad register to be used by the programmer to hold
data temporarily.
It is intended as a
Bit 2
PROGRAMMABLE BAUD RATE GENERATOR (AND
DIVISOR LATCHES DLH, DLL)
Trailing Edge of Ring Indicator (TERI). Bit 2 indicates
that the nRI input has changed from logic "0" to logic "1".
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 the
24 MHz crystal divided by 13, giving a 1.8462 MHz clock.
Bit 3
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
Table 33 shows the baud rates possible with a 1.8462
MHz crystal.
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.
Effect Of The Reset on Register File
Bit 7
The Reset Function Table (Table 34) details the effect of
the Reset input on each of the registers of the Serial Port.
This bit is the complement of the Data Carrier Detect
(nDCD) input. If bit 4 of the MCR is set
79
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