Intel® IXP42X Product Line of Network Processors and IXC1100 Control Plane Processor
September 2006 DM
Order Number: 252480-006US 339
Universal Asynchronous Receiver Transceiver (UART)—Intel® IXP42X product line and IXC1100
control plane processors
Leaving loop-back mode and returning to normal mode may result in unpredictable
activation of the Modem Status Register (MSR). It is recommended that the Modem
Status Register be read once to clear the Modem Status Bits in the Modem Status
Register.
In the loop-back diagnostic mode, data that is transmitted will be immediately
received. The ability to loop back the transmit data path to the receive path allows the
Intel XScale processor to verify the transmit data path and receive data path of the
UART. The transmit interrupts, receive interrupts, and modem-control interrupts are
operational, when placed in the loop-back diagnostic mode. The Modem-Control
Register Bits — instead of the Modem Control inputs — will activate the modem-control
interrupts. A break signal can also be transferred — from the transmitter section to the
receiver section — while operating in loop-back diagnostic mode.
Bit 1 of the Modem-Control Register is the Request-to-Send Bit. The Request-to-Send
Control Bit is used to program the RTS_N output pin. When the Request-to-Send
control bit is programmed to logic 0, the RTS_N signal will output logic 1. When the
Request-to-Send Control Bit is programmed to logic 1, the RTS_N signal will output
logic 0.
The Modem-Status Register is used to monitor the status of an external modem or data
set. The Modem Status Register is an 8-bit register that is used to detect when a
modem is capable of accepting new data. The status of the modem to accept incoming
data is monitored by reading the Clear-to-Send Bit of the Modem Status Register and
the Delta Clear-to-Send Bit of the Modem Status Register. The other six bits of the
Modem-Status Register can be used for UART debug purposed only.
Modem Status Register Bit 4 is the Clear-to-Send (CTS). The Clear-to-Send Bit will be
the complement of the Clear-to-Send (CTS_N) input signal. The Clear-to-Send signal
will be connected to the Ready-to-Send bit of the Modem Control Register when the
LOOP bit — in the Modem Control Register — is set to logic 1.
• CTS = logic 0 = CTS_N pin is 1
• CTS = logic 1 = CTS_N pin is 0
Modem Status Register Bit 0 is the Delta Clear-to-Send (DCTS). The Delta Clear-to-
Send bit will inform the IXP42X product line and IXC1100 control plane processors that
nothing has happened to the Clear-to-Send Status since the last time that the Modem-
Status Register was read. The Delta Clear-to-Send bit will be set to logic 0 after a read
of the Modem-Status Register.
• DCTS = logic 0 = No change in CTS_N pin since last read of the Modem-Status
Register
• DCTS = logic 1 = CTS_N pin has changed state since the last read of the Modem-
Status Register
The Modem-Control Register is initialized to hexadecimal 0x00 after reset. The Modem-
Status Register is initialized to hexadecimal 0x00 after reset.
10.2.4 UART Interrupts
The UART Inter rupt Enable Register (IER ) is an 8-bit register that enables five types of
UART based interrupts and enables the UART functionality and other control
functionality not used by the IXP42X product line and IXC1100 control plane
processors.
UART Interrupt Enable Register bit 6 is the UART Unit Enable (UUE) bit. When the UART
Unit Enable bit is set to logic 0, the UART will be completely non-functional. Likewise,
when the UART Unit Enable bit is set to logic 1, the UART will be enabled.