MCP2155_13 datasheet(12/52 Pages) MICROCHIP

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MCP2155

DS21690B-page 12

Preliminary

 2001-2013 Microchip Technology Inc.

2.9.1.2

IrLAP

The MCP2155 supports the IrLAP protocol. The IrLAP

protocol provides:

• Management of communication processes on the

link between devices.

• A device-to-device connection for the reliable,

ordered transfer of data.

• Device discover procedures.

• Hidden node handling. (Not supported by

MCP2155)

Figure 2-7 identifies the key parts and hierarchy of the

IrDA protocols. The bottom layer is the Physical layer,

IrPHY. This is the part that converts the serial data to

and from pulses of IR light. IR transceivers can’t trans-

mit and receive at the same time. The receiver has to

wait for the transmitter to finish sending. This is some-

times referred to as a “Half-Duplex” connection. The IR

Link Access Protocol (IrLAP) provides the structure for

packets or “frames” of data to emulate data that would

normally be free to stream back and forth.

FIGURE 2-7:

IRDA STANDARD

PROTOCOL LAYERS

Figure 2-8 shows how the IrLAP frame is organized.

The frame is proceeded by some number of Beginning

of Frame characters,(BOFs). The value of the BOF is

generally 0xC0, but 0xFF may be used if the last BOF

character is a 0xC0. The purpose of multiple BOFs is to

give the other station some warning that a frame is

coming.

The IrLAP frame begins with an address byte (“A”

field), then a control byte (“C” field). The control byte is

used to differentiate between different types of frames

and is also used to count frames. Frames can carry sta-

tus, data, or commands. The IrLAP protocol has a com-

mand syntax of it’s own, and these commands are part

of the control byte. Lastly, IrLAP frames carry data. This

data is the information or “I” field. The integrity of the

frame is ensured with a 16-bit CRC, referred to as the

Frame Check Sequence (FCS). The 16-bit CRC value

is transmitted LSB first. The end of the frame is marked

with an EOF character which is always a 0xC1. The

frame structure described here is used for all versions

of IrDA protocols used for serial wire replacement for

speeds up to 115.2 kbaud.

FIGURE 2-8:

IRLAP FRAME

In addition to defining the frame structure, IrLAP pro-

vides the “housekeeping” function of opening and clos-

ing connections, and maintaining connections once

they’re open. The critical parameters that determine

the performance of the link are part of this function.

These parameters control how many BOFs are used,

identify the speed of the link, how fast either party may

change from receiving to transmitting, etc. IrLAP has

the responsibility of negotiating these parameters to

the highest common set so that both sides can commu-

nicate as fast and as reliably as possible.

Host O.S. or Application

IrCOMM

IrLAP

IrPHY

Protocols

resident in

MCP2155

IR pulses

transmitted

and

received

IrLMP

–

IAS

Note 1: Another IrDA standard which is entering

general usage is IR Object Exchange

(IrOBEX). This standard is not used for

serial connection emulation.

2: IrDA communication standards faster

than 115.2 kbaud use a different CRC

method and physical layer.

X BOFs BOF A C

FCS

EOF

(1+N) of C0h payload

bytes C1h

21690B.book Page 12 Thursday, January 10, 2013 1:06 PM

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