PIC16F872_06 Datasheet, PDF (78/168 Pages) Microchip Technology – 28-Pin, 8-Bit CMOS Flash Microcontroller with 10-Bit A/D

PIC16F872
9.2.18.2 Bus Collision During a Repeated
START Condition
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.
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
SDA is low, a bus collision has occurred (i.e., another
master is attempting to transmit a data’0’). If, however,
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.
If, however, 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.
If, at the end of the BRG time-out, both SCL and SDA
are still high, the SDA pin is driven low, 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 9-23).
FIGURE 9-23:
BUS COLLISION DURING A REPEATED START CONDITION (CASE 1)
SDA
SCL
RSEN
Sample SDA when SCL goes high.
If SDA = 0, set BCLIF and release SDA and SCL.
BCLIF
S
'0'
SSPIF ^'0'
Cleared in software
'0'
'0'
FIGURE 9-24:
BUS COLLISION DURING REPEATED START CONDITION (CASE 2)
TBRG
TBRG
SDA
SCL
BCLIF
RSEN
SCL goes low before SDA,
set BCLIF. Release SDA and SCL.
Interrupt cleared
in software
S
'0'
'0'
SSPIF
'0'
'0'
DS30221C-page 76
© 2006 Microchip Technology Inc.