TL16C550D_09 Datasheet, PDF (37/54 Pages) Texas Instruments – ASYNCHRONOUS COMMUNICATIONS ELEMENT WITH AUTOFLOW CONTROL

TL16C550D, TL16C550DI
www.ti.com .................................................................................................................................................. SLLS597E – APRIL 2004 – REVISED DECEMBER 2008
• Bit 7: This bit is the divisor latch access bit (DLAB). Bit 7 must be set to access the divisor latches of the baud
generator during a read or write. Bit 7 must be cleared during a read or write to access the receiver buffer,
the THR, or the IER.
Line Status Register (LSR)⁽¹⁾
The LSR provides information to the CPU concerning the status of data transfers. The contents of this register
are summarized in Table 3 and described in the following bulleted list.
• Bit 0: This bit is the data ready (DR) indicator for the receiver. DR is set whenever a complete incoming
character has been received and transferred into the RBR or the FIFO. DR is cleared by reading all of the
data in the RBR or the FIFO.
• Bit 1⁽²⁾: This bit is the overrun error (OE) indicator. When OE is set, it indicates that before the character in the
RBR was read, it was overwritten by the next character transferred into the register. OE is cleared every time
the CPU reads the contents of the LSR. If the FIFO mode data continues to fill the FIFO beyond the trigger
level, an overrun error occurs only after the FIFO is full, and the next character has been completely received
in the shift register. An overrun error is indicated to the CPU as soon as it happens. The character in the shift
register is overwritten, but it is not transferred to the FIFO.
• Bit 2⁽³⁾: 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 TL16C550D 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.
(1) The line status register is intended for read operations only; writing to this register is not recommended outside of a factory testing
environment.
(2) Bits 1 through 4 are the error conditions that produce a receiver line status interrupt.
(3) Bits 1 through 4 are the error conditions that produce a receiver line status interrupt.
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