PIC18F242 datasheet(153/332 Pages) MICROCHIP

153 / 332 page

DS39564C-page 151

PIC18FXX2

15.4.7

BAUD RATE GENERATOR

In I2C Master mode, the baud rate generator (BRG)

reload value is placed in the lower 7 bits of the

SSPADD register (Figure 15-17). When a write occurs

to SSPBUF, the baud rate generator will automatically

begin counting. The BRG counts down to 0 and stops

until another reload has taken place. The BRG count is

decremented twice per instruction cycle (TCY) on the

Q2 and Q4 clocks. In I2C Master mode, the BRG is

reloaded automatically.

Once the given operation is complete (i.e., transmis-

sion of the last data bit is followed by ACK), the internal

clock will automatically stop counting and the SCL pin

will remain in its last state.

Table 15-3 demonstrates clock rates based on

instruction cycles and the BRG value loaded into

SSPADD.

FIGURE 15-17:

BAUD RATE GENERATOR BLOCK DIAGRAM

TABLE 15-3:

I2C CLOCK RATE W/BRG

SSPM3:SSPM0

BRG Down Counter

CLKO

Fosc/4

SSPADD<6:0>

SSPM3:SSPM0

SCL

Reload

Control

Reload

FCY

FCY*2

BRG Value

FSCL(2)

(2 Rollovers of BRG)

10 MHz

20 MHz

19h

400 kHz(1)

10 MHz

20 MHz

20h

312.5 kHz

10 MHz

20 MHz

3Fh

100 kHz

4 MHz

8 MHz

0Ah

400 kHz(1)

4 MHz

8 MHz

0Dh

308 kHz

4 MHz

8 MHz

28h

100 kHz

1 MHz

2 MHz

03h

333 kHz(1)

1 MHz

2 MHz

0Ah

100kHz

1 MHz

2 MHz

00h

1 MHz(1)

Note 1: The I2C interface does not conform to the 400 kHz I2C specification (which applies to rates greater than

100 kHz) in all details, but may be used with care where higher rates are required by the application.

2: Actual frequency will depend on bus conditions. Theoretically, bus conditions will add rise time and extend

low time of clock period, producing the effective frequency.

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