
88
IR Transmit Pin
The following description pertains to the TXD2/IRTX pin of the LPC47B37x.
The LPC47B37x implements a configuration bit to control state of the TXD2/IRTX pin. The
TXD2_MODE bit, bit 7 of the Serial Port 2 Mode configuration register (at 0xF0 in LD5), controls the
operation of the TXD2/IRTX pin as described below. The TXD2_MODE bit is a VTR powered bit that is
only reset on VTR POR.
If the TXD2_MODE bit is 0, then following a VTR POR, the TXD2/IRTX pin will be output and low. It will
remain low until one of the following conditions are met:
GP53/TXD2/IRTX Pin. This pin defaults to the GPIO function.
1. This pin will remain low following a VCC POR until the TXD2/IRTX function is selected for the pin
AND serial port 2 is enabled by setting the activate bit, at which time the pin will reflect the state of
the IR transmit output of the IR block (if IR is enabled through the IR Option Register for Serial Port
2).
2. This pin will remain low following a VCC POR until the TXD2 function is selected for the pin AND
serial port 2 is enabled by setting the activate bit, at which the pin will reflect the state of the
transmit output of serial port 2.
3. This pin will remain low following a VCC POR until the corresponding GPIO data bit (GP5 register
bit 3) is set or the polarity bit in the GP53 control register is set.
When Serial Port 2 is subsequently deactivated, the TXD2/IRTX pin will be driven low.
Following a VCC POR, setting the TXD2_MODE bit to 1 will tristate the TXD2/IRTX pin. This pin will
tristate regardless of the function selected on the pin (i.e., GPIO or TXD2 or IRTX), regardless of the
state of the activate bit for Serial Port 2 and regardless of the state of VCC. When VCC is removed
from the part, the TXD2/IRTX pin will remain tristate until a VTR POR occurs, which will reset the
TXD2_MODE bit.
This bit is implemented since there is a potential wakeup issue resulting from the operation of the TXD2
pin driving to zero when the UART2 is deactivated. The issue is that some modems do not assert the
ring indicator pin for wakeup if it senses the TXD2 pin as low. If required, this bit should be used as
follows:
When the activate bit for serial port 2 is cleared prior to entering a sleep state, set the
TXD2_MODE bit.
When the activate bit for serial port 2 is set upon to exiting a sleep state, clear the TXD2_MODE
bit.
The TXD2_MODE bit is defined as follows:
TXD2_MODE bit: Serial Port 2 Mode configuration register (0xF0 in LD5) bit 7, reset on VTR POR:
0=The inactive state of the TXD2/IRTX pin is low (default).
1=The GP53/TXD2/IRTX pin is tristate.
The inactive state of the TXD1 pin is tristate.