MCP2155_13 datasheet(15/52 Pages) MICROCHIP

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Preliminary

DS21690B-page 15

MCP2155

2.9.3

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 protocol is to allow this half-duplex link

to emulate, as much as possible, a full-duplex connec-

tion. 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 col-

lision. The rules of how and when these packets are

sent constitute the IrDA protocols. The MCP2155 sup-

ports elements of this IrDA protocol to communicate

with other IrDA standard compatible devices.

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 connec-

tion. These are:

• Normal Disconnect Mode (NDM)

• Discovery Mode

• Normal Connect Mode (NCM)

Figure 2-10 shows the connection sequence.

2.9.3.1

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 defaut 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 feature, 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 cellphones have IrDA standard infrared

ports. If you used such a cellphone with a Personal Dig-

ital Assistants (PDAs), the PDA that supports the IrDA

standard feature would be the Primary device and the

cellphone would be the Secondary device.

When a Primary device polls for another device, then 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. After all the packets are sent, the Pri-

mary device then 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 it

responds to determines the “time slot” to be used by

the Secondary device. For example, if the Secondary

device responds after packet number 2, then the Sec-

ondary device will use time slot 2. If the Secondary

device responds after packet number 0, then the Sec-

ondary device will use time slot 0. This mechanism

allows the Primary device to recognize as many nearby

devices as there are time slots. The Primary device will

continue to generate time slots and the Secondary

device should continue to respond, even if there’s noth-

ing to do.

During NDM, the MCP2155 handles all of the

responses to the Primary device (see Figure 2-10),

without any communication with the Host controller.

The Host controller is inhibited by the CTS signal, of the

MCP2155, from sending data to the MCP2155.

Note 1: The MCP2155 can only be used to

implement a Secondary device.

2: The MCP2155 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 MCP2155 always takes time slot 2.

4: If another Secondary device is nearby,

the Primary device may fail to recognize

the MCP2155, or the Primary device may

not recognize either of the devices.

21690B.book Page 15 Thursday, January 10, 2013 1:06 PM

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