Datasheet

MCP2120
DS21618B-page 6 © 2007 Microchip Technology Inc.
2.4.1.2 Software Selection
When the BAUD2:BAUD0 pins are configured as ’111’
the MCP2120 defaults to a baud rate of F
OSC / 768.
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
Command
(3, 4)
Frequency (MHz)
Bit Rate
0.6144
(1)
2.000 3.6864 4.9152 7.3728 14.7456
(2)
20.000
(2)
0x87
800 2604 4800 6400 9600 19200 26042 FOSC / 768
0x8B
1600 5208 9600 12800 19200 38400 52083 FOSC / 384
0x85
3200 10417 19200 25600 38400 78600 104167 FOSC / 192
0x83
4800 15625 28800 38400 57600 115200 156250 FOSC / 128
0x81
9600 31250 57600 78600 115200 230400 312500 FOSC / 64
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.