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TL16C550C, TL16C550CI
ASYNCHRONOUS COMMUNICATIONS ELEMENT
WITH AUTOFLOW CONTROL
SLLS177F
–
MARCH 1994
–
REVISED MARCH 2001
30
POST OFFICE BOX 655303
DALLAS, TEXAS 75265
PRINCIPLES OF OPERATION
line status register (LSR) (continued)
Bit 2
: This bit is the parity error (PE) indicator. When PE is set, it indicates that the parity of the received
data character does not match the parity selected in the LCR (bit 4). PE is cleared every time the CPU reads
the contents of the LSR. In the FIFO mode, this error is associated with the particular character in the FIFO
to which it applies. This error is revealed to the CPU when its associated character is at the top of the FIFO.
Bit 3
: This bit is the framing error (FE) indicator. When FE is set, it indicates that the received character
did not have a valid (set) stop bit. FE is cleared every time the CPU reads the contents of the LSR. In the
FIFO mode, this error is associated with the particular character in the FIFO to which it applies. This error
is revealed to the CPU when its associated character is at the top of the FIFO. The ACE tries to
resynchronize after a framing error. To accomplish this, it is assumed that the framing error is due to the
next start bit. The ACE samples this start bit twice and then accepts the input data.
Bit 4
: This bit is the break interrupt (BI) indicator. When BI is set, it indicates that the received data input
was held low for longer than a full-word transmission time. A full-word transmission time is defined as the
total time to transmit the start, data, parity, and stop bits. BI is cleared every time the CPU reads the contents
of the LSR. In the FIFO mode, this error is associated with the particular character in the FIFO to which it
applies. This error is revealed to the CPU when its associated character is at the top of the FIFO. When a
break occurs, only one 0 character is loaded into the FIFO. The next character transfer is enabled after SIN
goes to the marking state for at least two RCLK samples and then receives the next valid start bit.
Bit 5: This bit is the THRE indicator. THRE is set when the THR is empty, indicating that the ACE is ready
to accept a new character. If the THRE interrupt is enabled when THRE is set, an interrupt is generated.
THRE is set when the contents of the THR are transferred to the TSR. THRE is cleared concurrent with the
loading of the THR by the CPU. In the FIFO mode, THRE is set when the transmit FIFO is empty; it is cleared
when at least one byte is written to the transmit FIFO.
Bit 6: This bit is the transmitter empty (TEMT) indicator. TEMT bit is set when the THR and the TSR are
both empty. When either the THR or the TSR contains a data character, TEMT is cleared. In the FIFO mode,
TEMT is set when the transmitter FIFO and shift register are both empty.
Bit 7: In the TL16C550C mode, this bit is always cleared. In the TL16C450 mode, this bit is always cleared.
In the FIFO mode, LSR7 is set when there is at least one parity, framing, or break error in the FIFO. It is
cleared when the microprocessor reads the LSR and there are no subsequent errors in the FIFO.
modem control register (MCR)
The MCR is an 8-bit register that controls an interface with a modem, data set, or peripheral device that is
emulating a modem. The contents of this register are summarized in Table 3 and are described in the following
bulleted list.
Bit 0: This bit (DTR) controls the DTR output.
Bit 1: This bit (RTS) controls the RTS output.
Bit 2: This bit (OUT1) controls OUT1, a user-designated output signal.
Bit 3: This bit (OUT2) controls OUT2, a user-designated output signal.
When any of bits 0 through 3 are set, the associated output is forced low. When any of these bits are cleared,
the associated output is forced high.
The line status register is intended for read operations only; writing to this register is not recommended outside of a factory testing environment.
Bits 1 through 4 are the error conditions that produce a receiver line status interrupt.