PIC18CXX2_13 Datasheet, PDF (145/304 Pages) Microchip Technology – High Performance Microcontrollers with 10-bit A/D

PIC18CXX2
14.4.16.1 Bus Collision During a START
Condition
During a START condition, a bus collision occurs if:
a) SDA or SCL are sampled low at the beginning of
the START condition (Figure 14-24).
b) SCL is sampled low before SDA is asserted low
(Figure 14-25).
During a START condition, both the SDA and the SCL
pins are monitored.
If the SDA pin is already low, or the SCL pin is already
low, then all of the following occur:
• the START condition is aborted,
• the BCLIF flag is set, and
• the MSSP module is reset to its IDLE state
(Figure 14-24).
The START condition begins with the SDA and SCL
pins de-asserted. When the SDA pin is sampled high,
the baud rate generator is loaded from SSPADD<6:0>
and counts down to 0. If the SCL pin is sampled low
while SDA 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 SDA pin is sampled low during this count, the
BRG is reset and the SDA line is asserted early
(Figure 14-26). If, however, a '1' is sampled on the SDA
pin, the SDA pin is asserted low at the end of the BRG
count. The baud rate generator is then reloaded and
counts down to 0, and during this time, if the SCL pins
are sampled as '0', a bus collision does not occur. At
the end of the BRG count, the SCL 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 SDA before the
other. This condition does not cause a bus
collision, because the two masters must be
allowed to arbitrate the first address follow-
ing 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 14-24: BUS COLLISION DURING START CONDITION (SDA ONLY)
SDA
SDA goes low before the SEN bit is set.
. Set BCLIF,
S bit and SSPIF set because
SDA = 0, SCL = 1.
SCL
SEN
BCLIF
S
Set SEN, enable START
condition if SDA = 1, SCL=1
SDA sampled low before
START condition. Set BCLIF.
S bit and SSPIF set because
SDA = 0, SCL = 1.
SEN cleared automatically because of bus collision.
SSP module reset into idle state.
SSPIF and BCLIF are
cleared in software
SSPIF
SSPIF and BCLIF are
cleared in software
 1999-2013 Microchip Technology Inc.
DS39026D-page 145