PIC18CXX2
14.3.16.2 BUS COLLISION DURING A REPEATED
START CONDITION
If SDA is low, a bus collision has occurred (i.e. another
master, Figure 14-27, is attempting to transmit a data
’0’). If SDA is sampled high, the BRG is reloaded and
begins counting. If SDA goes from high to low before
the BRG times out, no bus collision occurs because no
two masters can assert SDA at exactly the same time.
During a Repeated Start condition, a bus collision
occurs if:
a) A low level is sampled on SDA when SCL goes
from low level to high level.
If SCL goes from high to low before the BRG times out
and SDA has not already been asserted, a bus collision
occurs. In this case, another master is attempting to
transmit a data ’1’ during the Repeated Start condition,
Figure 14-28.
b) SCL goes low before SDA is asserted low, indi-
cating that another master is attempting to trans-
mit a data ’1’.
When the user de-asserts SDA and the pin is allowed
to float high, the BRG is loaded with SSPADD<6:0>
and counts down to 0. The SCL pin is then de-asserted,
and when sampled high, the SDA pin is sampled.
If at the end of the BRG time out both SCL and SDA are
still high, the SDA pin is driven low and the BRG is
reloaded and begins counting. At the end of the count,
regardless of the status of the SCL pin, the SCL pin is
driven low and the Repeated Start condition is com-
plete.
FIGURE 14-27: BUS COLLISION DURING A REPEATED START CONDITION (CASE 1)
SDA
SCL
Sample SDA when SCL goes high.
If SDA = 0, set BCLIF and release SDA and SCL
RSEN
BCLIF
Cleared in software
'0'
S
'0'
SSPIF
FIGURE 14-28: BUS COLLISION DURING REPEATED START CONDITION (CASE 2)
TBRG
TBRG
SDA
SCL
SCL goes low before SDA,
BCLIF
RSEN
Set BCLIF. Release SDA and SCL
Interrupt cleared
in software
’0’
S
SSPIF
DS39026B-page 148
Preliminary
7/99 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
