23
ST16C552/552A
Rev. 3.40
LSR BIT-2:
Logic 0 = No parity error. (normal default condition)
Logic 1 = Parity error. The receive character does not
have correct parity information and is suspect. In the
FIFO mode, this error is associated with the character
at the top of the FIFO.
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). In the FIFO mode this error is
associated with the character at the top of the FIFO.
LSR BIT-4:
Logic 0 = No break condition. (normal default condi-
tion)
Logic 1 = The receiver received a break signal (RX
was a logic 0 for one character frame time). In the
FIFO mode, only one break character is loaded into
the FIFO.
LSR BIT-5:
This bit is the Transmit Holding Register Empty indi-
cator. This bit indicates that the UART is ready to
accept a new character for transmission. In addition,
this bit causes the UART to issue an interrupt to CPU
when the THR interrupt enable is set. The THR bit is
set to a logic 1 when a character is transferred from the
transmit holding register into the transmitter shift
register. The bit is reset to logic 0 concurrently with the
loading of the transmitter holding register by the CPU.
In the FIFO mode this bit is set when the transmit FIFO
is empty; it is cleared when at least 1 byte is written to
the transmit FIFO.
LSR BIT-6:
This bit is the Transmit Empty indicator. This bit is set
to a logic 1 whenever the transmit holding register and
the transmit shift register are both empty. It is reset to
logic 0 whenever either the THR or TSR contains a
data character. In the FIFO mode this bit is set to one
whenever the transmit FIFO and transmit shift register
are both empty.
LSR BIT-7:
Logic 0 = No Error. (normal default condition)
Logic 1 = At least one parity error, framing error or
break indication is in the current FIFO data. This bit is
cleared when RHR register is read.
Modem Status Register (MSR)
This register provides the current state of the control
interface signals from the modem, or other peripheral
device that the 552/552A is connected to. Four bits of
this register are used to indicate the changed informa-
tion. 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 552/552A 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 552/552A 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 552/552A 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:
Normally MSR bit-4 bit is the compliment of the -CTS
input. However 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.
