PIC18C658 MICROCHIP [Microchip Technology], PIC18C658 Datasheet - Page 163

• Manufacturer Part Number: PIC18C658
• Description: High-Performance Microcontrollers with CAN Module
• Manufacturer: MICROCHIP [Microchip Technology]

15.4.16.1 Bus Collision During a START Condition
During a START condition, a bus collision occurs if:
a)
b)
During a START condition, both the SDA and the SCL
pins are monitored.
If:
then:
FIGURE 15-21: BUS COLLISION DURING START CONDITION (SDA ONLY)
 2000 Microchip Technology Inc.
SDA
SCL
SEN
BCLIF
S
SSPIF
SDA or SCL are sampled low at the beginning of
the START condition (Figure 15-21).
SCL is sampled low before SDA is asserted low
(Figure 15-22).
the SDA pin is already low
or the SCL pin is already low,
the START condition is aborted,
and the BCLIF flag is set,
and the MSSP module is reset to its IDLE state
(Figure 15-21).
Set SEN, enable START
condition if SDA = 1, SCL = 1.
SDA sampled low before
START condition.
S bit and SSPIF set because
SDA = 0, SCL = 1.
. Set BCLIF,
SDA goes low before the SEN bit is set.
S bit and SSPIF set because
SDA = 0, SCL = 1.
Advanced Information
Set BCLIF.
SSPIF and BCLIF are
cleared in software.
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 15-23). 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 pin is
sampled as ’0’, a bus collision does not occur. At the
end of the BRG count, the SCL pin is asserted low.
Note:
SEN cleared automatically because of bus collision.
SSP module reset into IDLE state.
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.
SSPIF and BCLIF are
cleared in software.
PIC18CXX8
DS30475A-page 163