PIC18F4550ESOSQTP datasheet(230/438 Pages) MICROCHIP

230 / 438 page

PIC18F2455/2550/4455/4550

DS39632E-page 228

19.4.7

BAUD RATE

In I2C Master mode, the Baud Rate Generator (BRG)

reload value is placed in the lower seven bits of the

SSPADD register (Figure 19-19). 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 19-3 demonstrates clock rates based on

instruction cycles and the BRG value loaded into

SSPADD. SSPADD values of less than 2 are not

supported. Due to the need to support I2C clock

stretching capability, I2C baud rates are partially

dependent upon system parameters, such as line

capacitance and pull-up strength. The parameters

provided in Table 19-3 are guidelines, and the actual

baud rate may be slightly slower than that predicted in

the table. The baud rate formula shown in the bit

description of Register 19-4 sets the maximum baud

rate that can occur for a given SSPADD value.

FIGURE 19-19:

BAUD RATE GENERATOR BLOCK DIAGRAM

TABLE 19-3:

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 Rollovers of BRG)

10 MHz

20 MHz

18h

400 kHz(1)

10 MHz

20 MHz

1Fh

312.5 kHz

10 MHz

20 MHz

63h

100 kHz

4 MHz

8 MHz

09h

400 kHz(1)

4 MHz

8 MHz

0Ch

308 kHz

4 MHz

8 MHz

27h

100 kHz

1 MHz

2 MHz

02h

333 kHz(1)

1 MHz

2 MHz

09h

100 kHz

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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