MCP2155_13 datasheet(16/52 Pages) MICROCHIP

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MCP2155

DS21690B-page 16

Preliminary

 2001-2013 Microchip Technology Inc.

2.9.3.2

Discovery Mode

Discovery mode allows the Primay device to determine

the capabilities of the Secondary device. The discovery

mode is entered after the MCP2155 (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 device and

a Secondary device can stay in NDM indefinitely.

When the Primary device has something to do, it then

initiates Discovery. Discovery has two parts. these are:

• Link initialization

• Resource determination

The first step is for the Primary device and Secondary

device to determine and then adjust to each other’s

hardware capabilities. These capabilities are parame-

ters like:

•Data rate

• Turn around time

• Number of packets without a response

• How long to wait before disconnecting

Both the Primary device and Secondary device begin

communications at 9600 baud, which is the default

baud rate. The Primary device sends its parameters,

then the Secondary device responds with its parame-

ters. For example, if the Primary supports all data rates

up to 115.2 kbaud and the Secondary device only sup-

ports 19.2 kbaud then the link will be established at

19.2 kbaud.

After the hardware parameters are established, the Pri-

mary device must determine if the Secondary device

has the resources it requires. If the Primary device has

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

printer, not a modem or other device. This determina-

tion 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 MCP2155 handles all of

the 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

MCP2155, from sending data to the MCP2155.

2.9.3.3

Normal Connect Mode (NCM)

After discovery has been completed, the Primary

device and MCP2155 (Secondary device) can freely

exchange data.

The MCP2155 can receive IR data or serial data, but

not both at the same time. The MCP2155 uses a hard-

ware handshake to stop the local serial port from send-

ing data while the MCP2155 is receiving IR data.

Both the Primary device and MCP2155 (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 device and Secondary

device will still exchange packets just to make sure that

the connection hasn’t unexpectedly been dropped.

When the Primary device has finished, it then transmits

the close link command to the MCP2155 (Secondary

device). The MCP2155 will confirm the close link com-

mand, and then both the Primary device and MCP2155

(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. This is the same as if

the data was being received by the host controller from

a UART.

Note:

The MCP2155 is limited to a data rate of

115.2 kbaud.

Note:

Data loss will result if this hardware hand-

shake 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 MCP2155 will revert to the NDM state

10 seconds after the last frame has been

received.

21690B.book Page 16 Thursday, January 10, 2013 1:06 PM

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