PIC18F2455_09 Datasheet, PDF (230/438 Pages) Microchip Technology – 28/40/44-Pin,High-Performance,Enhanced Flash,USB Microcontrollers with nanoWatt Technology

PIC18F2455/2550/4455/4550
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
SSPM3:SSPM0
SSPADD<6:0>
SSPM3:SSPM0
SCL
Reload
Control
Reload
CLKO
BRG Down Counter
FOSC/4
TABLE 19-3: I²C™ CLOCK RATE W/BRG
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
4 MHz
20 MHz
8 MHz
63h
100 kHz
09h
400 kHz⁽¹⁾
4 MHz
8 MHz
0Ch
308 kHz
4 MHz
1 MHz
8 MHz
2 MHz
27h
100 kHz
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.
DS39632E-page 228
© 2009 Microchip Technology Inc.