138 Hardware Reference Manual
Intel® IXP2800 Network Processor
Intel XScale® Core
3.12.3 UART Operation
The format of a UART data frame is shown in Figure31.
Figure 31. UART Data Frame
Each data frame is between 7 bits and 12 bits long depending on the size of data programmed, if
parity is enabled and if two stop bits is selected. The frame begins with a start bit that is represented
by a high to low transition. Next, either 5 to 8 bits of data are transmitted, beginning with the least
significant bit. An optional parity bit follows, which is set if even parity is enabled and an odd
number of ones exist within the data byte, or if odd parity is enabled and the data byte contains an
even number of ones. The data frame ends with one, one and a half, or two stop bits as programmed
by the user, which is represented by one or two successive bit periods of a logic one.

3.12.3.1 UART FIFO OPERATION

The UART has one transmit FIFO and one receive FIFO. The transmit FIFO is 64 bytes deep and
eight bits wide. The receive FIFO is 64 bytes deep and 11 bits wide.
3.12.3.1.1 UART FIFO Interrupt Mode Operation –
Receiver Interrupt
When the Receive FIFO and receiver interrupts are enabled (UART_FCR[0]=1 and
UART_IER[0]=1), receiver interrupts occur as follows:
•The receive data available interrupt is invoked when the FIFO has reached its programmed
trigger level. The interrupt is cleared when the FIFO drops below the programmed trigger
level.
•The UART_IIR receive data available indication also occurs when the FIFO trigger level is
reached, and like the interrupt, the bits are cleared when the FIFO drops below the trigger
level.
•The receiver line status interrupt (UART_IIR = C6H), as before, has the highest priority. The
receiver data available interrupt (UART_IIR=C4H) is lower. The line status interrupt occurs
only when the character at the top of the FIFO has errors.
•The data ready bit (DR in UART_LSR register) is set to 1 as soon as a character is transferred
from the shift register to the Receive FIFO. This bit is reset to 0 when the FIFO is empty.
B1741-02
Start
Bit Data
<0> Data
<1> Data
<2> Data
<3> Data
<4> Data
<5> Data
<6> Data
<7> Parity
Bit Stop
Bit 1 Stop
Bit 2
TXD or RXD pin
Notes:
Receive data sample counter frequency = 16x bit frequency, each bit is sampled three times in the middle.
Shaded bits are optional and can be proammed by users.
MSBLSB