ST16C450
16
Rev. 4.20
LSR BIT-3:
Logic 0 = No framing error (normal default condition).
Logic 1 = Framing error. The receive character did not
have a valid stop bit(s).
LSR BIT-4:
Logic 0 = No break condition (normal default condi-
tion)
Logic 1 = The receiver received a break signal.
LSR BIT-5:
This bit indicates that the ST16C450 is ready to accept
new characters for transmission. This bit causes the
ST16C450 to issue an interrupt to the CPU when the
transmit holding register is empty and the interrupt
enable is set.
Logic 0 = Transmit holding register is not empty.
(normal default condition)
Logic 1 = Transmit holding register is empty. When
this bit is a logic 1, the CPU can load a new characters
into the Transmit Holding Register for transmission.
LSR BIT-6:
Logic 0 = Transmitter holding and shift registers are
full.
Logic 1 = Transmitter holding and shift registers are
empty.
LSR BIT-7:
Not used and set to “0”.
Modem Status Register (MSR)
This register provides the current state of the control
interface signals from the modem, or other peripheral
device that the ST16C450 is connected to. Four bits
of this register are used to indicate the changed
information. These bits are set to a logic 1 whenever
a control input from the modem changes state. These
bits are set to a logic 0 whenever the CPU reads this
register.
MSR BIT-0:
Logic 0 = No -CTS Change (normal default condition)
Logic 1 = The -CTS input to the ST16C450 has
changed state since the last time it was read. A
modem Status Interrupt will be generated.
MSR BIT-1:
Logic 0 = No -DSR Change (normal default condition)
Logic 1 = The -DSR input to the ST16C450 has
changed state since the last time it was read. A
modem Status Interrupt will be generated.
MSR BIT-2:
Logic 0 = No -RI Change (normal default condition)
Logic 1 = The -RI input to the ST16C450 has changed
from a logic 0 to a logic 1. A modem Status Interrupt
will be generated.
MSR BIT-3:
Logic 0 = No -CD Change (normal default condition)
Logic 1 = Indicates that the -CD input to the has
changed state since the last time it was read. A
modem Status Interrupt will be generated.
MSR BIT-4:
CTS (active high, logical 1). Normally this bit is the
compliment of the -CTS input. In the loop-back mode,
this bit is equivalent to the RTS bit in the MCR register.
MSR BIT-5:
DSR (active high, logical 1). Normally this bit is the
compliment of the -DSR input. In the loop-back mode,
this bit is equivalent to the DTR bit in the MCR register.
MSR BIT-6:
RI (active high, logical 1). Normally this bit is the
compliment of the -RI input. In the loop-back mode this
bit is equivalent to the OP1 bit in the MCR register.
MSR BIT-7:
CD (active high, logical 1). Normally this bit is the
compliment of the -CD input. In the loop-back mode
this bit is equivalent to the OP2 bit in the MCR register.
