PIC18F258-ESLASHSOSQTP datasheet(171/402 Pages) MICROCHIP

171 / 402 page

DS41159E-page 169

PIC18FXX8

17.4.7

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 17-17). 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 17-3

demonstrates

clock

rates

based

instruction cycles and the BRG value loaded into

SSPADD.

FIGURE 17-17:

BAUD RATE GENERATOR BLOCK DIAGRAM

TABLE 17-3:

I2C™ CLOCK RATE w/BRG

SSPM3:SSPM0

BRG Down Counter

CLKO

FOSC/4

SSPADD<6:0>

SSPM3:SSPM0

SCL

Reload

Control

Reload

FOSC

FCY

FCY * 2

BRG Value

FSCL

(2 Rollovers of BRG)

40 MHz

10 MHz

20 MHz

18h

400 kHz(1)

40 MHz

10 MHz

20 MHz

1Fh

312.5 kHz

40 MHz

10 MHz

20 MHz

63h

100 kHz

16 MHz

4 MHz

8 MHz

09h

400 kHz(1)

16 MHz

4 MHz

8 MHz

0Ch

308 kHz

16 MHz

4 MHz

8 MHz

27h

100 kHz

4 MHz

1 MHz

2 MHz

02h

333 kHz(1)

4 MHz

1 MHz

2 MHz

09h

100kHz

4 MHz

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.

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