MCP2122

DS21894B-page 8

Preliminary

 2004 Microchip Technology Inc.

2.4.1

BAUD RATE

The baud rate for the MCP2122 is determined by the

frequency of the 16XCLK signal. Equation 2-1

demonstrates how to calculate the 16XCLK frequency

based on the desired baud rate. Table 2-3 shows some

common baud rates and the corresponding 16XCLK

frequency.

EQUATION 2-1:

16XCLK FREQUENCY

TABLE 2-3:

COMMON BAUD RATE/

16XCLK FREQUENCY

2.5

Encoder/Decoder

The IR encoder/decoder is made up of two major

components. They are:

• IR Decoder

• IR Encoder

The encoder receives UART data (bit by bit) and

outputs a data bit in the IrDA standard bit format.

Figure 2-8 shows a functional block diagram of the

encoder.

The decoder receives IrDA standard data (bit by bit)

and outputs data in UART data bit format. Figure 2-8

shows a functional block diagram of the decoder.

The encoder/decoder has two interfaces. They are:

• Host UART interface

•IR interface

2.5.1

ENCODING (MODULATION)

Each bit time is comprised of 16 bit clocks. If the value

to be transmitted (as determined by the TX pin) is a

logic-low, the TXIR pin will output a low level for 7-bit

clock cycles, a logic-high level for 3-bit clock (with a

maximum high-time of about 4 µs) cycles, with the

remaining time (6-bit clock cycles or more) being low. If

the value to transmit is a logic-high, the TXIR pin will

output a low level for the entire 16 bit clock cycle.

2.5.2

DECODING (DEMODULATION)

Each bit time is comprised of 16 bit clocks. If the value

to be received is a logic-low, the RXIR pin will be a low

level for the first 3-bit clock cycle (or a minimum of

1.6 µs), with the remaining time (13-bit clock cycles)

being high. If the value to be received is a logic-high,

the RXIR pin will be a high level for the entire 16-bit

clock cycle. The level on the RX pin will be in the

appropriate state for an entire 16-bit clock cycle.

FIGURE 2-8:

MCP2122 RECEIVE DETECT TO ENCODER/DECODER BLOCK DIAGRAM

Baud

Rate

16XCLK

Frequency

Comment

9600

153,600

19,200

307,200

38,400

614,400

57,600

921,600

115,200

1,843,200

16 (Desired Baud Rate)

•

=

TXIR

Glitch

Filter

Decode

RX

RXIR

Pulse Width

Limiter

Encode

TX

The following table shows the state on the RESET pin and how this effects the operation of the TXIR pin.

RESET State

Comment

TXIR output encoded value of TX pin

TXIR is forced low

(~ 4 µs)