3V to 5.5V, IrDA Infrared Transceiver with

Integrated RS-232 Interface

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11

IR Transmitter

The infrared transmitter consists of an internal high-

power, open-drain MOSFET switch. This switch has an

on-resistance of less than 2

Ω and is capable of switch-

ing 200mA of current. Internal buffering keeps the input

capacitance of the TXD pin extremely low to ease user

drive requirements. Connect an IR LED in series with a

current-setting resistor to select the appropriate IR out-

put power (see the

Powering the IR LED section). The

transmitter is not current limited so do not exceed the

power dissipation of the external components during

high duty-cycle transmit schemes.

The TXD input controls the IR LED for the MAX3131.

The T2IN input controls the IR LED for the MAX3130

(IRMODE = low). With the ENDEC disabled (see

IrDA

Encoder/Decoder (ENDEC) section), the IR LED is

turned on by a logic-high signal at the TXD or T2IN

input, for the MAX3131 and MAX3130 respectively.

IRMODE: Multiplexed RS-232 Operation

and IrDA Operation (MAX3130)

The MAX3130 has the capability to multiplex R2OUT and

T2IN between the IrDA infrared interface and the RS-232

electrical interface. The state of the IRMODE input deter-

mines which interface (infrared or RS-232) is multiplexed

to R2OUT and T2IN. When IRMODE is low, R2OUT acts

as the infrared receiver output and T2IN acts as the

infrared transmitter input. Also, while IRMODE is low, the

RS-232 charge pumps are shut down and the RS-232

transmitters are disabled (see

Shutdown section). When

IRMODE is high, R2OUT and T2IN assume their func-

tions as the RS-232 data receive output and transmit

input, respectively. Also, while IRMODE is high, the IR

transmitter is disabled (turned off).

EDGEDET: Edge-Detection Circuitry

(MAX3130)

The MAX3130 has internal edge-detection circuitry that

monitors the RS-232 R2OUT line when IRMODE is low

and monitors the IrDA receive channel when IRMODE

is high. EDGEDET goes low when a positive or negative

edge is detected on either the RS-232 R2OUT line or

the IrDA receive channel (depending on the IRMODE

pin). This edge-detection feature is useful for initiating

an interrupt when data is received on the deselected

line. The EDGEDET signal is cleared when IRMODE is

toggled. Table 1 shows EDGEDET operation.

IrDA Encoder/Decoder (ENDEC)

The MAX3130 and MAX3131 provide an on-board

ENDEC to communicate with UARTs that are not IrDA

compatible. The ENDEC is enabled by applying a clock

with a frequency 16 times the baud rate to the BAUD16

input. This BAUD16 clock is commonly provided on

UARTs that do not have IrDA ENDEC capability. Figure

3 illustrates the operation of the ENDEC. The ENDEC

stretches the incoming infrared pulse (a pulse between

IRSD

RSSD IRMODE

R2IN

IrDA RxIN

EDGEDET*

X

X

0

X

X

X

0

X

X

X

1

X

X

X

1

X

1.41

µs < t < 3CS

16CS

32CS

16CS

R2OUT (RXD)

WITH ENDEC ENABLED

R2OUT (RXD)

WITH ENDEC DISABLED

CS = BAUD16 CLOCK CYCLES

* HIGH = INFRARED LIGHT PULSE

( ) ARE FOR MAX3131

INFRARED

PHOTODIODE INPUT *

Figure 3a. ENDEC Operation, Receiving Infrared

X

= Don’t care

* EDGEDET

is cleared by any transition on IRMODE.