PIC18F44J50-I Datasheet, PDF (318/562 Pages) Microchip Technology – 28/44-Pin, Low-Power, High-Performance USB Microcontrollers

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19.5.17.1 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 19-28).
b) SCLx is sampled low before SDAx is asserted
low (Figure 19-29).
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 19-28)
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 19-30). 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.
Note:
The reason that 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
master 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.
FIGURE 19-28:
SDAx
BUS COLLISION DURING START CONDITION (SDAx ONLY)
SDAx goes low before the SEN bit is set.
Set BCLxIF,
S bit and SSPxIF set because
SDAx = 0, SCLx = 1.
SCLx
SEN
BCLxIF
S
Set SEN, enable Start
condition if SDAx = 1, SCLx = 1
SDAx sampled low before
Start condition. Set BCLxIF.
S bit and SSPxIF set because
SDAx = 0, SCLx = 1.
SEN cleared automatically because of bus collision.
MSSPx module reset into Idle state.
SSPxIF and BCLxIF are
cleared in software
SSPxIF
SSPxIF and BCLxIF are
cleared in software
DS39931D-page 318
 2011 Microchip Technology Inc.