PIC18F87J10 Datasheet, PDF (221/394 Pages) Microchip Technology – 64/80-Pin High-Performance, 1-Mbit Flash Microcontrollers with nanoWatt Technology

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18.4.17.1 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 18-26).
b) SCLx is sampled low before SDAx is asserted
low (Figure 18-27).
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 MSSP module is reset to its Idle state
(Figure 18-26).
The Start condition begins with the SDAx and SCLx
pins deasserted. When the SDAx pin is sampled high,
the Baud Rate Generator 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 18-28). 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 ‘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 factor
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 colli-
sion 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 18-26:
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.
SSP module reset into Idle state.
SSPxIF and BCLxIF are
cleared in software
SSPxIF
SSPxIF and BCLxIF are
cleared in software
 2005 Microchip Technology Inc.
Advance Information
DS39663A-page 219