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PDA 05 5/30/03 11:35 AM Page 69
Chapter 5 / The Electronics
packets (or “frames”) of data to emulate data that would normally be
free to stream back and forth.
IRDA Standard Protocol Layers
The IrLAP 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 mul-
tiple 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 status, data, or commands. The IrLAP has a com-
mand syntax of its own. These commands are part of the control byte.
Last, IrLAP frames carry data. These data are 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 trans-
mitted least signification bit (LSB) first. The end of the frame is
marked with an end of frame (EOF) character, which is always a 0xC1.
The frame structure described here is used for all versions of IrDA pro-
tocols used for serial wire replacement for speeds up to 115.2 kbaud.
In addition to defining the frame structure, IrLAP provides the “house-
keeping” functions of opening, closing, and maintaining connections.
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 communicate as quickly, and as reliably, as possible. This is
done during the handshaking phase when the PDA is connecting to
PDA Robot.

IrLMP. The MCP2150 implements the IrLMP. The IrLMP provides:

• Multiplexing of the IrLAP layer. This allows multiple channels
above an IrLAP connection.

• Protocol and service discovery via the IAS.
When two devices that contain the IrDA standard feature are connected,
generally one device has something to do and the other device has the

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