PIC16F872_06 Datasheet, PDF (76/168 Pages) Microchip Technology – 28-Pin, 8-Bit CMOS Flash Microcontroller with 10-Bit A/D

PIC16F872
9.2.18.1 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 9-20).
b) SCL is sampled low before SDA is asserted low.
(Figure 9-21).
During a START condition, both the SDA and the SCL
pins are monitored. If either the SDA pin or the SCL pin
is already low, then these events all occur:
• the START condition is aborted,
• and the BCLIF flag is set
• and the SSP module is reset to its IDLE state
(Figure 9-20).
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 9-22). 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. 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 9-20:
SDA
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.
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
DS30221C-page 74
© 2006 Microchip Technology Inc.