MCP2140-ISO datasheet(19/58 Pages) MICROCHIP

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 2003 Microchip Technology Inc.

Preliminary

DS21790A-page 19

MCP2140

2.14.2

DISCOVERY MODE

Discovery mode allows the Primary device to deter-

mine the capabilities of the MCP2140 (Secondary

device). Discovery mode is entered once the MCP2140

(Secondary device) has sent a XID response to the Pri-

mary device and the Primary device has completed

sending the XIDs and a Broadcast 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, which 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

• Turnaround time

• Number of packets without a response

• How long to wait before disconnecting

Both the Primary and Secondary devices begin com-

munications at 9600 baud, the default baud rate. The

Primary device sends its parameters and the Second-

ary device responds with its parameters. 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, it must know if it’s talking to a printer, and

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 que-

ries 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 MCP2140 handles all

responses to the Primary device (see Figure 2-13)

without any communication with the Host Controller.

The Host Controller is inhibited by the CTS signal of the

MCP2140 from sending data to the MCP2140.

2.14.3

NORMAL CONNECT MODE (NCM)

Once discovery has been completed, the Primary

device and MCP2140 (Secondary device) can freely

exchange data.

The MCP2140 uses a hardware handshake to stop the

local serial port from sending data when the MCP2140

Host UART Receiving buffer is full..

Both the Primary device and the MCP2140 (Secondary

device) check to make sure that data packets are

received by the other without errors. Even when data is

not 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 transmits the “close

link” command to the MCP2140 (Secondary device).

The MCP2140 will confirm the “close link” command

and both the Primary device and the MCP2140 (Sec-

ondary 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.

2.14.3.1

Primary Device Notification

The MCP2140 identifies itself to the Primary device as

a modem.

However, the MCP2140 is not a modem, and the non-

data circuits are not handled in a modem fashion.

Note:

The MCP2140 is limited to a data rate of

9.6 kbaud.

Note:

Data loss will result if this hardware

handshake is not observed.

Note:

If the NCM mode is unexpectedly termi-

nated for any reason (including the Primary

device not issuing a close link command),

the MCP2140 will revert to the NDM state

approximately 10 seconds after the last

frame has been received.

Note:

The MCP2140 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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