MCP2140AT-ISLASHP datasheet(49/60 Pages) MICROCHIP

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DS22050A-page 49

MCP2140A

B.2

Discovery Mode

Discovery

mode

allows

the

Primary

Device

determine the capabilities of the MCP2140A (Second-

ary Device). Discovery mode is entered once the

MCP2140A (Secondary Device) has sent an XID

response to the Primary Device and the Primary

Device has completed sending the XIDs and a Broad-

cast ID. If this sequence is not completed, a Primary

and Secondary Device can stay in NDM indefinitely.

When the Primary Device has something to do, it

initiates Discovery. Discovery has two parts. They are:

• Link initialization

• Resource determination

The first step is for the Primary and Secondary Devices

to determine, and then adjust to, each other’s hardware

capabilities. These capabilities are parameters like:

• Data rate

• Turn around time

• Number of packets without a response

• How long to wait before disconnecting

Both the Primary and Secondary Devices begin

communications at 9600 baud, which is the default

baud rate. The Primary Device sends its parameters

and the Secondary Device responds with its parame-

ters. For example, if the Primary Device supports all

data rates up to 115.2 kbaud and the Secondary

Device only supports 9.6 kbaud, the link will be

established at 9.6 kbaud.

Once the hardware parameters are established, the

Primary Device must determine if the Secondary

device has the resources it requires. If the Primary

Device has a job to print, then it must know if it’s talking

to a printer, not a modem or other device. This

determination is made using the Information Access

Service (IAS). The job of the Secondary Device is to

respond to IAS queries made by the Primary Device.

The Primary Device must ask a series of questions like:

• What is the name of your service?

• What is the address of this service?

• What are the capabilities of this device?

When all the Primary Device’s questions are answered,

the Primary Device can access the service provided by

the Secondary Device.

During Discovery mode, the MCP2140A handles all

responses to the Primary Device (see Figure B-1) with-

out any communication with the Host Controller. The

Host Controller is inhibited by the CTS signal of the

MCP2140A from sending data to the MCP2140A.

B.3

Normal Connect Mode (NCM)

Once discovery has been completed, the Primary

Device and Secondary Device can freely exchange

data.

The MCP2140A uses a hardware handshake to stop

the local serial port from sending data when the

MCP2140A Host UART Receiving buffer is full.

Both the Primary Device and the MCP2140A (Second-

ary Device) check to make sure that data packets are

received by the other without errors. Even when data is

required to be sent, the Primary and Secondary

Devices will still exchange packets to ensure that the

connection hasn’t, unexpectedly, been dropped. When

the Primary Device has finished, it then transmits the

close link command to the MCP2140A (Secondary

Device). The MCP2140A will confirm the “close link”

command and both the Primary Device and the

MCP2140A (Secondary Device) will revert to the NDM

state.

It is the responsibility of the Host Controller program to

understand the meaning of the data received and how

the program should respond to it. It’s just as if the data

were being received by the Host Controller from a

UART.

6.1.0.1

Primary Device Notification

The MCP2140A identifies itself to the Primary Device

as a modem.

However, the MCP2140A is not a modem, and the non-

data circuits are not handled in a modem fashion.

Note:

The MCP2140A is limited to a data rate of

9.6 kbaud.

Note:

Data loss will result if this hardware

handshake is not observed.

Note:

the

NCM

mode

unexpectedly

terminated for any reason (including the

Primary Device not issuing a close link

command), the MCP2140A will revert to

the NDM state after a time delay (after the

last frame has been received).

Note:

The MCP2140A identifies itself as a

modem to ensure that it is identified as a

serial device with a limited amount of

memory.

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