MCP2120

DS21618B-page 6

© 2007 Microchip Technology Inc.

2.4.1.2

Software Selection

When the BAUD2:BAUD0 pins are configured as ’111’

To place the MCP2120 into Command Mode, the

MODE pin must be at a low level. When in this mode,

any data that is received by the MCP2120’s UART is

"echoed" back to the controller and no encoding/

decoding occurs. The echoed data will be skewed less

than 1 bit time (see parameter IR141). When the

MODE pin goes high, the device is returned to Data

Mode where the encoder/decoder is in operation.

Table 2-2 shows the software hex commands to config-

ure the MCP2120’s baud rate.

The MCP2120 receives data bytes at the existing baud

rate. When the change baud rate command (0x11) is

received, the last valid baud rate value received

becomes the new baud rate. The new baud rate is

effective after the stop bit of the echoed data.

Figure 2-2 shows this sequence.

2.4.2

TRANSMITTING

When the controller sends serial data to the MCP2120,

the baud rates are required to match.

There will be some jitter on the detection of the high to

low edge of the start bit. This jitter will affect the place-

ment of the encoded start bit. All subsequent bits will be

16 BITCLK times later.

2.4.3

RECEIVING

When the controller receives serial data from the

MCP2120, the baud rates are required to match.

There will be some jitter on the detection of the high to

low edge of the start bit. This jitter will affect the place-

ment of the decoded Start bit. All subsequent bits will

be 16 BITCLK times later.

FIGURE 2-1:

Data/Command Mode

Flow

TABLE 2-2:

SOFTWARE BAUD RATE SELECTION VS. FREQUENCY

MODE pin goes low

Data Mode

Command Mode

Controller sends baud

MCP2120 echoes baud

Controller sends 0x11

MCP2120 echoes 0x11

New baud rate

MODE pin goes high

Data Mode

When echoing the Data, once the first bit is

detected, it is echoed back. This means that

the echoed data is skewed no more than 1 bit

time.

The new baud rate can occur once the echoed

stop bit completes.

Hex

Frequency (MHz)

Bit Rate

2.000

3.6864

4.9152

7.3728

0x87

800

2604

4800

6400

9600

19200

26042

0x8B

1600

5208

9600

12800

19200

38400

52083

0x85

3200

10417

19200

25600

38400

78600

104167

0x83

4800

15625

28800

38400

57600

115200

156250

0x81

9600

31250

57600

78600

115200

230400

312500

Note 1: An external clock is recommended for frequencies below 2 MHz.

2: For frequencies above 7.3728 MHz, the TXIR pulse width (parameter IR121) will be shorter than the 1.6 µs

IrDA standard specification.

3: Command 0x11 is used to change to the new baud rate.

4: All other command codes are reserved for possible future use.