TL16C2552 Datasheet, PDF (27/34 Pages) Texas Instruments – 1.8-V to 5-V DUAL UART WITH 16-BYTE FIFOS

TL16C2552
www.ti.com
SLWS163A – SEPTEMBER 2005 – REVISED JUNE 2006
• 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)
NOTE:
The line status register is intended for read operations only; writing to this register is
not recommended outside of a factory testing environment.
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.
NOTE:
Bits 1 through 4 are the error conditions that produce a receiver line status interrupt.
• Bit 1: 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: 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 RX
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 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.
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