Slow AMBA Peripherals
13-14
GMS30C7201 Data Sheet
Bit 1:
This bit is the Overrun Error (OE) indicator. Bit 1 indicates that data in
the Receiver Buffer Register was not read by the CPU before the next
character was transferred into the Receiver Buffer Register, thereby
destroying the previous character. The OE indicator is set to a logic 1
upon detection of an overrun condition and reset whenever the CPU
reads the contents of the Line Status Register. If the FIFO mode data
continues to fill the FIFO beyond the trigger level, an overrun error will
occur only after the FIFO is full and the next character has been
completely received in the shift register. OE 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.
This bit is the Parity Error (PE) indicator. Bit 2 indicates that the
received data character does not have the correct even or odd parity,
as selected by the even-parity-select bit. The PE bit is set to a logic 1
upon detection of a parity error and is reset to a logic 0 whenever the
CPU reads the contents of the Line Status Register. In the FIFO mode,
this error is associated with the particular character in the FIFO it
applies to. This error is revealed to the CPU when its associated
character is at the top of the FIFO.
This bit is the Framing Error (FE) indicator. Bit 3 indicates that the
received character did not have a valid stop bit. Bit 3 is set to a logic 1
whenever the Stop bit following the last data bit or parity bit is detected
as a logic 0 bit (Spacing level). The FE indicator is reset whenever the
CPU reads the contents of the Line Status Register. In the FIFO mode
this error is associated with the particular character in the FIFO it
applies to. This error is revealed to the CPU when its associated
character is at the top of the FIFO. The UART will try to re-synchronize
after a framing error. To do this it assumes that the framing error was
due to the next start bit, so it samples this
“
start
”
bit twice and then
takes in the
“
data
”
.
This bit is the Break Interrupt (BI) indicator. Bit 4 is set to a logic 1
whenever the received data input is held in the Spacing (logic 0) state
for longer than a full word transmission time (that is, the total time of
Start bit + data bits + Parity + Stop bits). The BI indicator is reset
whenever the CPU reads the contents of the Line Status Register. In
the FIFO mode this error is associated with the particular character in
associated character is at the top of the FIFO. When break occurs,
only one zero character is loaded into the FIFO. The next character
transfer is enabled after
SIN
goes to the marking state and receives
the next valid start bit.
Note: Bits 1
—
4 are the error conditions that produce a Receiver Line
Status interrupt whenever any of the corresponding conditions are
detected and the interrupt is enabled.
This bit is the Transmitter Holding Register Empty (THRE) indicator. Bit
5 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 the CPU when the Transmit Holding Register Empty Interrupt enable
is set HIGH. The THRE bit is set to a logic 1 when a character is
transferred from the Transmitter 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 XMIT FIFO is empty; it is cleared when at least 1
byte is written to the XMIT FIFO.
Bit 2:
Bit 3:
Bit 4:
Bit 5: