MCP2140-IP datasheet(50/58 Pages) MICROCHIP

50 / 58 page

MCP2140

DS21790A-page 50

Preliminary

 2003 Microchip Technology Inc.

APPENDIX C: HOW DEVICES

CONNECT

When two devices implementing the IrDA standard fea-

ture establish a connection using the IrCOMM protocol,

the process is analogous to connecting two devices

with serial ports using a cable. This is referred to as a

“point-to-point” connection. This connection is limited

to half-duplex operation because the IR transceiver

cannot transmit and receive at the same time. The pur-

pose of the IrDA protocols is to allow this half-duplex

link to emulate, as much as possible, a full-duplex con-

nection. In general, this is done by dividing the data into

“packets”, or groups of data. These packets can then

be sent back and forth, when needed, without risk of

collision. The rules of how and when these packets are

sent constitute the IrDA protocols.

When a wired connection is used, the assumption is

made that both sides have the same communications

parameters and features. A wired connection has no

need to identify the other connector because it is

assumed that the connectors are properly connected.

In the IrDA standard, a connection process has been

defined to identify other IrDA compatible devices and

establish a communication link. There are three steps

that these two devices go through to make this

connection. They are:

• Normal Disconnect Mode (NDM)

• Discovery Mode

• Normal Connect Mode (NCM)

Figure C-1 shows the connection sequence.

C.1

Normal Disconnect Mode (NDM)

When two IrDA standard compatible devices come into

range they must first recognize each other. The basis

of this process is that one device has some task to

accomplish and the other device has a resource

needed to accomplish this task. One device is referred

to as a Primary device and the other is referred to as a

Secondary device. This distinction between Primary

device and Secondary device is important. It is the

responsibility of the Primary device to provide the

mechanism to recognize other devices. So the Primary

device must first poll for nearby IrDA standard compat-

ible devices. During this polling, the default baud rate of

9600 baud is used by both devices.

For example, if you want to print from an IrDA equipped

laptop to an IrDA printer, utilizing the IrDA standard fea-

ture, you would first bring your laptop in range of the

printer. In this case, the laptop is the one that has

something to do and the printer has the resource to do

it. The laptop is called the Primary device and the

printer is the Secondary device. Some data-capable

cell phones have IrDA standard infrared ports. If you

used such a cell phone with a Personal Digital Assis-

tant (PDA), the PDA that supports the IrDA standard

feature would be the Primary device and the cell phone

would be the Secondary device.

When a Primary device polls for another device, a

nearby Secondary device may respond. When a Sec-

ondary device responds, the two devices are defined to

be in the Normal Disconnect Mode (NDM) state. NDM

is established by the Primary device broadcasting a

packet and waiting for a response. These broadcast

packets are numbered. Usually 6 or 8 packets are sent.

The first packet is number 0, the last packet is usually

number 5 or 7. Once all the packets are sent, the Pri-

mary device sends an ID packet, which is not num-

bered.

The Secondary device waits for these packets and then

responds to one of the packets. The packet responds

to determines the “timeslot” to be used by the Second-

ary device. For example, if the Secondary device

responds after packet number 2, then the Secondary

device will use timeslot 2. If the Secondary device

responds after packet number 0, then the Secondary

device will use timeslot 0. This mechanism allows the

Primary device to recognize as many nearby devices

as there are timeslots. The Primary device will continue

to generate timeslots and the Secondary device should

continue to respond, even if there’s nothing to do.

Note 1: The MCP2140 can only be used to

implement a Secondary device.

2: The MCP2140 supports a system with

only one Secondary device having exclu-

sive use of the IrDA standard infrared link

(known as “point-to-point” communica-

tion).

3: The MCP2140 always responds to packet

number 2. This means that the MCP2140

will always use timeslot 2.

4: If another Secondary device is nearby,

the Primary device may fail to recognize

the MCP2140, or the Primary device may

not recognize either of the devices.

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