MCP2150 datasheet(16/52 Pages) MICROCHIP

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MCP2150

DS21655B-page 16

Preliminary

 2002 Microchip Technology Inc.

2.9.3.2

Discovery Mode

Discovery mode allows the Primary device to deter-

mine the capabilities of the MCP2150 (Secondary

device). Discovery mode is entered once the MCP2150

(Secondary device) has sent an XID response to the

Primary device and the Primary device has completed

sending the XIDs and then sends a Broadcast ID. If this

sequence is not completed, then 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 device begin commu-

nications at 9600 baud, which is the default baud rate.

The Primary device sends its parameters, then the

Secondary device responds with its parameters. For

example, if the Primary supports all data rates up to

115.2 kbaud and the Secondary device only supports

19.2 kbaud, the link will be established at 19.2 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 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 MCP2150 handles all

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

MCP2150 from sending data to the MCP2150.

2.9.3.3

Normal Connect Mode (NCM)

Once discovery has been completed, the Primary

device and MCP2150 (Secondary device) can freely

exchange data.

The MCP2150 can receive IR data or serial data, but

not both simultaneously. The MCP2150 uses a hard-

ware handshake to stop the local serial port from

sending data while the MCP2150 is receiving IR data.

Both the Primary device and the MCP2150 (Secondary

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 MCP2150 (Secondary

device). The MCP2150 will confirm the close link com-

mand and both the Primary device and the MCP2150

(Secondary device) will revert to the NDM state.

It is the responsability 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.

Note:

The MCP2150 is limited to a data rate of

115.2 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 MCP2150 will revert to the NDM state

10 seconds after the last frame has been

received.

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