MCP2140-IP datasheet(9/58 Pages) MICROCHIP

9 / 58 page

 2003 Microchip Technology Inc.

Preliminary

DS21790A-page 9

MCP2140

2.4

Host UART Interface

The Host UART interface communicates with the Host

Controller. This interface has eight signals associated

with it: TX, RX, RTS, CTS, DSR, DTR, CD and RI. Sev-

eral of these signals are locally generated (not passed

over the IR interface). The Host UART is a half-duplex

interface, meaning that the system is either transmitting

or receiving, but not both simultaneously.

2.4.1

BAUD RATE

The baud rate for the MCP2140 serial port (the TX and

RX pins) is fixed at 9600 baud when the device

frequency is 7.3728 MHz.

2.4.2

TRANSMITTING

When the controller sends serial data to the MCP2140,

the controller’s baud rate is required to match the baud

rate of the MCP2140’s serial port.

2.4.3

RECEIVING

When the controller receives serial data from the

MCP2140, the controller’s baud rate is required to

match the baud rate of the MCP2140’s serial port.

2.4.4

HARDWARE HANDSHAKING

There are three Host UART signals used to control the

handshaking operation between the Host Controller

and the MCP2140. They are:

•DSR

•RTS

•CTS

2.4.4.1

DSR

The DSR signal is used to indicate that a link has been

established between the MCP2140 and the Primary

Device. Please refer to Section 2.13, “How Devices

Connect”, for information on how devices connect.

2.4.4.2

RTS

The RTS signal indicates to the MCP2140 that the Host

Controller is ready to receive serial data. Once an IR

data packet has been received, the RTS signal will be

low for the received data to be transferred to the Host

Controller. If the RTS signal remains high, an IR link

timeout will occur and the MCP2140 will disconnect

from the Primary Device.

2.4.4.3

CTS

The MCP2140 generates the CTS signal locally due to

buffer limitations.

The MCP2140 uses a 64-byte buffer for incoming data

from the IR Host. Another 29-byte buffer is provided to

buffer data from the UART serial port. The MCP2140

can handle IR data and Host UART serial port data

simultaneously. A hardware handshaking pin (CTS) is

provided to inhibit the Host Controller from sending

serial data when the Host UART buffer is not available

(Figure 2-3). Figure 2-4 shows a flow chart for Host

UART flow control using the CTS signal.

FIGURE 2-3:

HOST UART CTS SIGNAL AND THE RECEIVE BUFFER

Note 1: The MCP2140 generates several non-

data signals locally.

2: The MCP2140 emulates a 3-wire serial

connection (TXD, RXD and GND). The

transceiver’s

Transmit

Data

(TXD),

Receive Data (RXD) signals, and the

state of the CD. RI and DTR input pins

are carried back and forth to the Primary

device.

3: The RTS and CTS signals are local

emulations.

Note:

When the CTS output signal goes high, the

UART FIFO will store up to 6 bytes. This is

to allow devices that have a slow response

time to a change on the CTS signal time to

stop sending additional data (such as a

modem).

CTS

Receive Buffer Empty

MCP2140 Can Receive Data Receive Buffer Has 22 Bytes,

Receive Buffer

Receive Buffer Empty

MCP2140 Can Receive Data

IR Data Packet Transmitted

Full (29 Bytes)

CTS Pin Driven High

IR Data Packet Starts Transmission

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