Datasheet

PIC16(L)F1938/9
DS40001574C-page 276  2011-2013 Microchip Technology Inc.
24.6.13.1 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 24-33).
b) SCL is sampled low before SDA is asserted low
(Figure 24-34).
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 24-33).
The Start condition begins with the SDA and SCL pins
deasserted. When the SDA pin is sampled high, the
Baud Rate Generator is loaded and counts down. If the
SCL pin is sampled low while SDA is high, a bus colli-
sion 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 24-35). 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 zero; if the SCL pin is sampled as ‘0’
during this time, a bus collision does not occur. At the
end of the BRG count, the SCL pin is asserted low.
FIGURE 24-33: BUS COLLISION DURING START CONDITION (SDA ONLY)
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 SDA before the
other. This condition does not cause a bus
collision because the two masters must be
allowed to arbitrate the first address fol-
lowing the Start condition. If the address is
the same, arbitration must be allowed to
continue into the data portion, Repeated
Start or Stop conditions.
SDA
SCL
SEN
SDA sampled low before
SDA goes low before the SEN bit is set.
S bit and SSPIF set because
SSP module reset into Idle state.
SEN cleared automatically because of bus collision.
S bit and SSPIF set because
Set SEN, enable Start
condition if SDA =
1, SCL = 1
SDA = 0, SCL = 1.
BCLIF
S
SSPIF
SDA = 0, SCL = 1.
SSPIF and BCLIF are
cleared by software
SSPIF and BCLIF are
cleared by software
Set BCLIF,
Start condition. Set BCLIF.