Hardware UART

17.3Signal Descriptions

Table 17-1lists and describes each external signal that is connected to the UART module. The pins are connected to the PXA255 processor through GPIOs.

Table 17-1. UART Signal Descriptions

Name

Type

Description

 

 

 

RXD

Input

SERIAL INPUT – Serial data input to the receive shift register. In infrared mode, it is connected to the infrared

 

 

receiver input.

TXD

Output

SERIAL OUTPUT – Serial data output to the communications link-peripheral, modem, or data set. The TXD

signal is set to the logic 1 state upon a Reset operation. It is connected to the output of the infrared transmitter in

 

 

infrared mode.

 

 

 

 

 

CLEAR TO SEND – When low, indicates that the modem or data set is ready to exchange data.

 

 

Non-Autoflow Mode: When not in autoflow mode, bit 4 (CTS) of the Modem Status register (MSR) indicates the

 

 

state of nCTS. Bit 4 is the complement of the nCTS signal. Bit 0 (DCTS) of the Modem Status register indicates

 

 

whether the nCTS input has changed state since the previous reading of the Modem Status register. When the

 

 

CTS bit of the MSR changes state and the modem status interrupt is enabled, an interrupt is generated. nCTS

 

 

has no effect on the transmitter. The user can program the UART to interrupt the processor when DCTS changes

 

 

state. The programmer can then stall the outgoing data stream by starving the transmit FIFO or disabling the

nCTS

Input

UART with the Interrupt Enable register (IER).

 

 

NOTE: If UART transmission is stalled by disabling the UART, the user will not receive an MSR interrupt when

 

 

nCTS reasserts. This is because disabling the UART also disables interrupts. To get around this, either

 

 

use Auto CTS in Autoflow Mode, or program the nCTS GPIO pin to interrupt.

 

 

Autoflow Mode: In autoflow mode, the UART transmit circuity checks the state of nCTS before transmitting each

 

 

byte. If nCTS is high, no data is transmitted.

 

 

 

 

 

REQUEST TO SEND – When low, signals the modem or the data set that the UART is ready to exchange data.

 

 

Non-Autoflow Mode: The nRTS output signal can be asserted by setting bit 1 (RTS) of the Modem Control

nRTS

Output

register to 1. The RTS bit is the complement of the nRTS signal.

 

 

Autoflow Mode: nRTS is automatically asserted by the autoflow circuitry when the receive buffer exceeds its

 

 

programmed trigger threshold. It is deasserted when enough bytes are removed from the buffer to lower the data

 

 

level back to the trigger threshold.

 

 

 

17.4 Operation

The format of a UART data frame is shown in Figure 17-1.

Figure 17-1. Example UART Data Frame

Start Data Data Data Bit <0> <1> <2>

TXD or RXD pin

Data Data Data Data Data Parity Stop Stop <3> <4> <5> <6> <7> Bit Bit 1 Bit 2

LSB

MSB

Intel® PXA255 Processor Developer’s Manual

17-3

Page 575
Image 575
Intel PXA255 manual Operation, Uart Signal Descriptions