PIC18F87J10 Datasheet, PDF (212/394 Pages) Microchip Technology – 64/80-Pin High-Performance, 1-Mbit Flash Microcontrollers with nanoWatt Technology

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18.4.7 BAUD RATE
In I²C Master mode, the Baud Rate Generator (BRG)
reload value is placed in the lower 7 bits of the
SSPxADD register (Figure 18-17). When a write
occurs to SSPxBUF, 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 SCLx pin
will remain in its last state.
Table 18-3 demonstrates clock rates based on
instruction cycles and the BRG value loaded into
SSPxADD.
18.4.7.1 Baud Rate and Module
Interdependence
Because MSSP1 and MSSP2 are independent, they
can operate simultaneously in I²C Master mode at
different baud rates. This is done by using different
BRG reload values for each module.
Because this mode derives its basic clock source from
the system clock, any changes to the clock will affect
both modules in the same proportion. It may be possi-
ble to change one or both baud rates back to a previous
value by changing the BRG reload value.
FIGURE 18-17:
BAUD RATE GENERATOR BLOCK DIAGRAM
SSPM3:SSPM0
SSPxADD<6:0>
SSPM3:SSPM0
SCLx
Reload
Control
Reload
CLKO
BRG Down Counter
FOSC/4
TABLE 18-3: 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⁾
Note 1:
1 MHz
2 MHz
09h
100 kHz
1 MHz
2 MHz
00h
1 MHz⁽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.
DS39663A-page 210
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