MCP2150-ISLASHSS datasheet(18/52 Pages) MICROCHIP

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MCP2150

DS21655B-page 18

Preliminary

 2002 Microchip Technology Inc.

2.10

Operation

The MCP2150 emulates a null modem connection. The

application on the DTE device sees a virtual serial port.

This serial port emulation is provided by the IrDA stan-

dard protocols. The link between the DTE device and

the embedded application is made using the

MCP2150. The connection between the MCP2150 and

the embedded application is wired as if there were a

null modem connection.

The Carrier Detect (CD) signal of the MCP2150 is used

to indicate that a valid IrDA standard infrared link has

been established between the MCP2150 and the Pri-

mary device. The CD signal should be monitored

closely to make sure that any communication tasks can

be completed. The MCP2150 DSR signal indicates that

the device has powered-up, successfully initialized and

is ready for service. This signal is intended to be con-

nected to the DSR input of the Host Controller. If the

Host Controller was directly connected to an IrDA stan-

dard Primary device using a serial cable (the MCP2150

is not present), the Host Controller would be connected

to the Primary device’s DTR output signal.

The MCP2150 generates the CTS signal locally

because of buffer limitations.

2.10.1

HARDWARE HANDSHAKING

The MCP2150 uses a 64-byte buffer for incoming data

from the IR Host. Another 64-byte buffer is provided to

buffer data from the UART serial port. When an IR

packet begins the IrComm, the MCP2150 handles IR

data exclusively (the UART serial port buffer is not

available). A hardware handshaking pin (CTS) is pro-

vided to inhibit the Host Controller from sending serial

data while IR Data is being sent or received.

2.10.2

BUFFERS AND THROUGHPUT

The maximum IR data rate of the MCP2150 is

115.2 kbaud. The actual throughput will be less, due to

several factors. The most significant factors are under

the control of the developer. One factor beyond the

control of the designer is the overhead associated with

the IrDA standard. The MCP2150 uses a fixed data

block size of 64 bytes. To carry 64 bytes of data, the

MCP2150 must send 72 bytes (64+8). The additional 8

bytes are used by the protocol. When the Primary

device receives the frame, it must wait for a minimum

latency period before sending a packet of its own. This

turnaround time is set by IrLAP when the parameters of

the link are negotiated. A common turnaround time is

1 ms, although longer and shorter times may be

encountered. 1 ms represents approximately 12 byte

times at a data rate of 115.2 kbaud. The minimum size

frame the Primary device can respond with is 6 bytes.

The MCP2150 will add the 12 byte-time latency on its

own, again assuming a 1 ms latency. This means that

the maximum throughput will be 64 data bytes out of a

total of 64 + 38 byte times. Thus, the maximum theoret-

ical throughput will be limited to about 64/(64+38)=63%

of the IR data rate. Actual maximum throughput will be

dependent

both

the

MCP2150

and

the

characteristics of the Primary device.

The most significant factor in data throughput is how

well the data frames are filled. If only 1 byte is sent at a

time, then the maximum throughput is 1/(1+38)=2.5%

of the IR data rate. The best way to maximize through-

put is to align the amounts of data with the packet size

of the MCP2150. Throughput examples are shown in

Table 2-4.

TABLE 2-4:

THEORETICAL IrDA STANDARD THROUGHPUT EXAMPLES @ 115.2 KBAUD

Note 1: The

MCP2150

generates

non-data

signals locally.

2: Only transceiver’s TXD and RXD signals

are carried back and forth to the Primary

device. The MCP2150 emulates a 3-wire

serial connection (TXD, RXD and GND).

Note:

When the CTS output from the IrComm is

high, no data should be sent from the Host

Controller. The UART FIFO will store up to

2 bytes. Any additional data bytes will be

lost.

Note:

IrDA throughput is based on many factors

associated with characteristics of the Pri-

mary and Secondary devices. These char-

acteristics may cause your application

throughput to be less than the theoretical

example shown in Table 2-4.

MCP2150

Data Packet

Size (Bytes)

Overhead

(Bytes)

Primary Device

Minimum

Response (Bytes)

Primary Device

Turn-around Time(1)

(Bytes)

MCP2150

Turn-around

Time(1) (Bytes)

Total Bytes

Transmitted

Throughput

% (Data/Total)

102

62.7%

2.6%

Note 1: Number of bytes calculated based on a common turnaround time of 1 ms.

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