PIC16F1847_13 Datasheet, PDF (274/440 Pages) Microchip Technology – 18/20/28-Pin Flash Microcontrollers with XLP Technology

PIC16(L)F1847
25.6.13.1 Bus Collision During a Start
Condition
During a Start condition, a bus collision occurs if:
a) SDAx or SCLx are sampled low at the beginning
of the Start condition (Figure 25-33).
b) SCLx is sampled low before SDAx is asserted
low (Figure 25-34).
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 and
• the MSSPx module is reset to its Idle state
(Figure 25-33).
The Start condition begins with the SDAx and SCLx
pins deasserted. When the SDAx pin is sampled high,
the Baud Rate Generator is loaded and counts down. 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 25-35). If, however, a ‘1’ is sampled on the
SDAx pin, the SDAx pin is asserted low at the end of
the BRG count. The Baud Rate Generator is then
reloaded and counts down to zero; 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
factor during a Start condition is that no
two bus masters can assert a Start condi-
tion 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 25-33:
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.
SSPx module reset into Idle state.
SSPxIF and BCLxIF are
cleared by software
SSPxIF
SSPxIF and BCLxIF are
cleared by software
DS40001453D-page 274
Preliminary
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