MCP2140-ISO datasheet(18/58 Pages) MICROCHIP

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MCP2140

DS21790A-page 18

Preliminary

 2003 Microchip Technology Inc.

2.14

How The MCP2140 Connects

When two devices, implementing the IrDA standard

feature, establish a connection using the IrCOMM pro-

tocol, 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 trans-

ceiver cannot transmit and receive at the same time.

The purpose of the IrDA standard protocol is to allow

this half-duplex link to emulate, as much as possible, a

full-duplex connection. In general, this is done by divid-

ing the data into “packets”, or groups of data. These

packets can be sent back and forth, when needed,

without risk of collision. The rules of how and when

these packets are sent constitute the IrDA standard

protocol. The MCP2140 supports elements of this IrDA

standard protocol to communicate with other IrDA stan-

dard 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.

According to the IrDA standard, a connection process

has been defined to identify other IrDA standard com-

patible 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 2-13 shows the connection sequence.

2.14.1

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 while the other is referred to as

a Secondary device. The distinction between Primary

device and Secondary device is important because 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 and, 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-equipped 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. Thus, 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 cell phone with a Personal

Digital Assistant (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, while the last packet

is usually numbered 5 or 7. Once all the packets are

sent, the Primary device sends an ID packet, which is

not numbered.

The Secondary device waits for these packets and then

responds to one of the packets. The packet responds

to determine the “timeslot” to be used by the Secondary

device. For example, if the Secondary device responds

after packet number 2, the Secondary device will use

timeslot 2. If the Secondary device responds after

packet number 0, 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.

During NDM, the MCP2140 handles all responses to

the Primary device (Figure 2-13) without any communi-

cation with the Host Controller. The Host Controller is

inhibited by the CTS signal of the MCP2140 from

sending data to the MCP2140.

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

exclusive use of the IrDA standard infra-

red

link

(known as

“point-to-point”

communication).

3: The MCP2140 always responds to packet

number 0. This means that the MCP2140

will always use timeslot 0.

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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