PIC17C75X
15.2.14.1 BUS COLLISION DURING A START
CONDITION
If the SDA pin is sampled low during this count, the
BRG is reset and the SDA line is asserted early
(Figure 15-40). 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
is sampled as '0', a bus collision does not occur. At
the end of the BRG count the SCL pin is asserted low.
During a START condition, a bus collision occurs if:
a) SDA or SCL are sampled low at the beginning of
the START condition (Figure 15-38)
b) SCL is sampled low before SDA is asserted low.
(Figure 15-39)
During a START condition both the SDA and the SCL
pins are monitored.
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, and if the
address is the same, arbitration must be
allowed to continue into the data portion,
RESTART, or STOP conditions.
If:
the SDA pin is already low
or the SCL pin is already low,
then:
the START condition is aborted,
and the BCLIF flag is set,
and the SSP module is reset to its IDLE state
(Figure 15-38).
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.
FIGURE 15-38: BUS COLLISION DURING START CONDITION (SDA ONLY)
SDA goes low before the SEN bit is set.
. Set BCLIF,
S bit and SSPIF set because
SDA = 0, SCL = 1
SDA
SCL
Set SEN, enable start
condition if SDA = 1, SCL=1
SEN cleared automatically because of bus collision.
SSP module reset into idle state.
SEN
SDA sampled low before
START condition.
Set BCLIF.
S bit and SSPIF set because
SDA = 0, SCL = 1
BCLIF
SSPIF and BCLIF are
cleared in software.
S
SSPIF
SSPIF and BCLIF are
cleared in software.
1997 Microchip Technology Inc.
Preliminary
DS30264A-page 161
MICROCHIP [ MICROCHIP ]
