PIC16F872 Datasheet, PDF (81/160 Pages) Microchip Technology – 28-Pin, 8-Bit CMOS FLASH Microcontroller

PIC16F872
9.2.18.16 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 deasserts 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 deasserted, 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 count-
ing. 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’
© 1999 Microchip Technology Inc.
Preliminary
DS30221A-page 81