Datasheet

PIC18F47J13 FAMILY
DS39974A-page 340 Preliminary  2010 Microchip Technology Inc.
20.5.17.1 Bus Collision During a Start
Condition
During a Start condition, a bus collision occurs if:
a) SDAx or SCLx is sampled low at the beginning
of the Start condition (Figure 20-28).
b) SCLx is sampled low before SDAx is asserted
low (Figure 20-29).
During a Start condition, both the SDAx and the SCLx
pins are monitored.
If the SDAx pin is already low, or the SCLx pin is
already low, then all of the following occur:
• The Start condition is aborted
• The BCLxIF flag is set
• The MSSP module is reset to its inactive state
(Figure 20-28)
The Start condition begins with the SDAx and SCLx
pins deasserted. When the SDAx pin is sampled high,
the BRG is loaded from SSPxADD<6:0> and counts
down to 0. If the SCLx pin is sampled low while SDAx
is high, a bus collision occurs because it is assumed
that another master is attempting to drive a data ‘1’
during the Start condition.
If the SDAx pin is sampled low during this count, the
BRG is reset and the SDAx line is asserted early
(Figure 20-30). If, however, a ‘1’ is sampled on the
SDAx pin, the SDAx pin is asserted low at the end of
the BRG count. The BRG is then reloaded and counts
down to 0. If the SCLx pin is sampled as ‘0’ during this
time, a bus collision does not occur. At the end of the
BRG count, the SCLx pin is asserted low.
FIGURE 20-28: BUS COLLISION DURING START CONDITION (SDAx ONLY)
Note: The reason that a bus collision is not a fac-
tor during a Start condition is that no two
bus masters can assert a Start condition at
the exact same time. Therefore, one mas-
ter will always assert SDAx before the
other. This condition does not cause a bus
collision because the two masters must be
allowed to arbitrate the first address
following the Start condition. If the address
is the same, arbitration must be allowed to
continue into the data portion, Repeated
Start or Stop conditions.
SDAx
SCLx
SEN
SDAx Sampled Low Before
SDAx goes Low Before the SEN bit is Set;
S bit and SSPxIF Set because
MSSPx module Reset into Idle State
SEN Cleared Automatically because of Bus Collision;
S bit and SSPxIF Set because
Set SEN, Enable Start
Condition if SDAx = 1, SCLx = 1
SDAx = 0, SCLx = 1
BCLxIF
S
SSPxIF
SDAx = 0, SCLx = 1
SSPxIF and BCLxIF are
Cleared in Software
SSPxIF and BCLxIF are
Cleared in Software
Set BCLxIF,
Start Condition; Set BCLxIF;