MCP2140AT-ISLASHP datasheet(21/60 Pages) MICROCHIP

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DS22050A-page 21

MCP2140A

2.10.2

IMPROVING THROUGHPUT

Actual maximum throughput is dependent on several

factors, including:

• Characteristics of the Primary device

• Characteristics of the MCP2140A

• IrDA standard protocol overhead

• Direction of Data Transmitted

2.10.2.1

Characteristics of the Primary

Device and the MCP2140A

While the characteristics of the MCP2140A are fixed,

the characteristics of the Primary Device may be made

better, or worse, depending on the implementation of

the Primary Device’s application program.

IrDA Standard protocol overhead limits the maximum

throughput to much less then the 9600 baud bit rate.

2.10.2.2

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. In the IrCOMM 9-wire

“cooked” service class, 4 bytes of the 64 byte payload

are overhead bytes. That leaves a maximum of

60 bytes that can be transferred per “frame”.

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 2-1). 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

MCP2140A.

TABLE 2-1:

IRCOMM OVERHEAD %

2.10.2.3

Direction of Data Transmitted

Due to the different characteristics of the Primary

Device and the Secondary Device, the data throughput

between two devices may be substantially different

depending on the direction the data is being trans-

ferred. This is shown in Table 2-4.

MCP2140A to Primary Device

One of the factors to improve throughput is to ensure

the Host Controller keeps the MCP2140A UART

Receive Buffer full (60 bytes) to maximize the data

bytes sent per IR frame. Figure 2-10 shows the CTS

waveform, what the state of the buffers can be and the

operation of the Host UART and IR interfaces.

A second is to minimize the turn around response time

from the Primary device, if possible.

Primary Device to MCP2140A

In this case, ensure that the Host Controller can receive

data as fast as possible from the MCP2140A without

requiring to force the RTS high and delay the data from

being received by the Host Controller.

2.10.2.4

From the Primary Device

The MCP2140A uses a fixed IR Receiver data block

size of 64 bytes.

The minimum size frame the Primary device can

respond with is 6 bytes.

2.10.2.5

From the MCP2140A

The MCP2140A uses a fixed Host UART Receiver data

block size of 60 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

recommended between the time a byte is received and

another byte is transmitted.

2.12

Device ID

The MCP2140A has a fixed Device ID. This Device ID

is “MCP2140A XX”, with the xx indicating the silicon

revision of the device.

MCP2140A

Data

Packet

Size

(Bytes)

IrCOMM

Overhead

(Bytes)

IrCOMM

Overhead

% (1)

Comment

IR Receive

12 %

Note 2

89 %

Host UART

Receive

12 %

Note 3,

Note 4

89 %

Note 1: Overhead % =

Overhead/(Overhead + Data).

2: The maximum number of data bytes in the

IR Receive frame.

3: The maximum number of bytes of the Host

UART Receive buffer.

4: The CTS signal is driven high after the 59

byte.

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