ST16C550 Exar Corporation, ST16C550 Datasheet - Page 19

• Manufacturer Part Number: ST16C550
• Description: UART WITH 16-BYTE FIFO
• Manufacturer: Exar Corporation

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 regis-
ter. 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 there are no remaining LSR errors in the
RX FIFO.
Modem Status Register (MSR)
This register provides the current state of the control
interface signals from the modem, or other peripheral
device that the ST16C550 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 ST16C550 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 ST16C550 has
changed state since the last time it was read. A
modem Status Interrupt will be generated.
Rev. 4.30
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MSR BIT-2:
Logic 0 = No -RI Change (normal default condition)
Logic 1 = The -RI input to the ST16C550 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.
Scratchpad Register (SPR)
The ST16C550 provides a temporary data register to
store 8 bits of user information.
ST16C550