MCP2140AT-ISLASHSS datasheet(48/60 Pages) MICROCHIP

48 / 60 page

MCP2140A

DS22050A-page 48

B.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

compatible 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

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 (Client) and

the printer is the Secondary Device (Server). Some

data-capable cell phones 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

Secondary Device responds, the two devices are

defined to be in the Normal Disconnect Mode (NDM)

state. NDM is established by the Primary Device broad-

casting 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 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 determines the “timeslot” to be used by the

Secondary 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.

During NDM, the MCP2140A handles all responses to

the

Primary

Device

(Figure B-1)

without

any

communication with the Host Controller. The Host Con-

troller is inhibited by the CTS signal of the MCP2140A

from sending data to the MCP2140A.

Note:

In the parlance of software development,

the Primary Device is called a Client and

the Secondary Device is called a Server.

Note 1: The MCP2140A can only be used to

implement a Secondary Device.

2: The MCP2140A supports a system with

only

one

Secondary

Device

having

exclusive use of the IrDA standard

infrared link (known as “point-to-point”

communication).

3: The MCP2140A always responds to

packet number 0. This means that the

MCP2140A will always use timeslot 0.

4: If another Secondary Device is nearby,

the Primary Device may fail to recognize

the MCP2140A, or the Primary Device

may not recognize either of the devices.

MCP2140AT-I/SS Datasheet(PDF) 48 Page - Microchip Technology

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