Intel PXA26X User Manual download pdf (Page 588)

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17-4 Intel® PXA26x Processor Family Developer’s Manual

Hardware UART

Receive data sample counter frequency is 16 times the value of the bit frequency. The 16X clock is

created by the baud rate generator. Each bit is sampled three times in the middle. Shaded bits in

Figure 17-1 are optional and can be programmed by software.

Each data frame is between seven and 12 bits long, depending on the size of the data programmed,

whether parity is enabled, and the number of stop bits. A data frame begins by transmitting a start

bit that is represented by a high to low transition. The start bit is followed by from five to eight bits

of data that begin with the least significant bit (LSB). The data bits are followed by an optional

parity bit. The parity bit is set if even parity is enabled and the data byte has an odd number of ones

or if odd parity is enabled and the data byte has an even number of ones. The data frame ends with

one, one and a half or two stop bits, as programmed by software. The stop bits are represented by

one, one and a half, or two successive bit periods of a logic one.

The UART has two FIFOs: one transmit and one receive. The transmit FIFO is 64 bytes deep and

eight bits wide. The receive FIFO is 64 bytes deep and 11 bits wide. Three bits are used for tracking

errors.

The UART can use NRZ coding to represent individual bit values. NRZ coding is enabled when the

Interrupt Enable Register’s (IER) bit 5, IER[5] is set to high. A one is represented by a line

transition and a zero is represented by no line transition. Figure 17-2 shows the data byte 0b 0100

1011 in NRZ coding. The byte’s LSB is transmitted first.

Figure 17-1. Example UART Data Frame

Start

Bit

Data

<0>

Data

<1>

Data

<2>

Data

<3>

Data

<4>

Data

<5>

Data

<6>

Data

<7>

Parit

Bit

Stop

Bit 1

Stop

Bit 2

TXD or RXD pin

LSB

MSB

Figure 17-2. Example NRZ Bit Encoding (0b0100 1011)

Digital

Data

NRZ

Data

Bit

Value

11010010

LSB

MSB

1 2 ... 583 584 585 586 587 588 589 590 591 592 593 ... 623 624

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