MCP2140-ISO datasheet(16/58 Pages) MICROCHIP

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MCP2140

DS21790A-page 16

Preliminary

 2003 Microchip Technology Inc.

2.10.1

IMPROVING THROUGHPUT

Actual maximum throughput is dependent on several

factors, including:

• Characteristics of the Primary device

• Characteristics of the MCP2140

• IrDA standard protocol overhead

The IrDA standard specifies how the data is passed

between the Primary device and Secondary device. In

IrCOMM, an additional 8 bytes are used by the protocol

for each packet transfer.

The most significant factor in data throughput is how

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

time, the throughput overhead of the IrCOMM protocol

is 89% (see Table 2-5). The best way to maximize

throughput is to align the amounts of data with the

receive buffer (IR and Host UART) packet size of the

MCP2140.

Then there is the delay between when data packets

are sent and received. See Figure 2-10 for an example

of this delay (look at CTS signal falling edges). In this

screen capture, a Palm™ m105 is receiving a 240-

byte string of data from the MCP2140. When the CTS

signal goes high, the Host Controller stops sending

data (23 bytes per CTS low-time). The CTS falling

edge to CTS falling edge is approximately 90 ms (typ-

ical). This CTS high-time affects the total data through-

put. The CTS high-time will be dependant on the

characteristics of the Primary device.

TABLE 2-5:

IRCOMM OVERHEAD %

2.10.1.1

From the Primary Device

The MCP2140 uses a fixed IR Receiver data block size

of 64 bytes.

The minimum size frame the Primary device can

respond with is 6 bytes.

2.10.1.2

From the MCP2140

The MCP2140 uses a fixed Host UART Receiver data

block size of 29 bytes.

2.11

Turnaround Latency

An IR link can be compared to a one-wire data connec-

tion. The IR transceiver can transmit or receive, but not

both at the same time. A delay of one bit time is recom-

mended between the time a byte is received and

another byte is transmitted.

2.12

Device ID

The MCP2140 has a fixed Device ID. This Device ID is

“MCP2140 xx”, with the xx indicating the silicon

revision of the device.

MCP2140

Data

Packet

Size

(Bytes)

IrCOMM

Overhead

(Bytes)

IrCOMM

Overhead

% (1)

Comment

Receive

11 %

Note 2

89 %

Host

UART

Receive

22 %

Note 3

26 %

Note 4

89 %

Note 1: Overhead % =

Overhead/(Overhead + Data).

2: The maximum number of bytes of the IR

Receive buffer.

3: The maximum number of bytes of the

Host UART Receive buffer.

4: The CTS signal is driven high at 23 byte.

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