Microchip Technology DV164136 Data Sheet
© 2008 Microchip Technology Inc.
DS39646C-page 243
PIC18F8722 FAMILY
19.4.17.2
Bus Collision During a Repeated
Start Condition
Start Condition
During a Repeated Start condition, a bus collision
occurs if:
occurs if:
a)
A low level is sampled on SDAx when SCLx
goes from low level to high level.
goes from low level to high level.
b)
SCLx goes low before SDAx is asserted low,
indicating that another master is attempting to
transmit a data ‘1’.
indicating that another master is attempting to
transmit a data ‘1’.
When the user deasserts SDAx and the pin is allowed
to float high, the BRG is loaded with SSPxADD<6:0>
and counts down to ‘0’. The SCLx pin is then
deasserted and when sampled high, the SDAx pin is
sampled.
to float high, the BRG is loaded with SSPxADD<6:0>
and counts down to ‘0’. The SCLx pin is then
deasserted and when sampled high, the SDAx pin is
sampled.
If SDAx is low, a bus collision has occurred (i.e., another
master is attempting to transmit a data ‘0’, Figure 19-29).
If SDAx is sampled high, the BRG is reloaded and
begins counting. If SDAx goes from high-to-low before
the BRG times out, no bus collision occurs because no
two masters can assert SDAx at exactly the same time.
master is attempting to transmit a data ‘0’, Figure 19-29).
If SDAx is sampled high, the BRG is reloaded and
begins counting. If SDAx goes from high-to-low before
the BRG times out, no bus collision occurs because no
two masters can assert SDAx at exactly the same time.
If SCLx goes from high-to-low before the BRG times
out and SDAx 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 (see Figure 19-30).
out and SDAx 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 (see Figure 19-30).
If, at the end of the BRG time-out, both SCLx and SDAx
are still high, the SDAx pin is driven low and the BRG is
reloaded and begins counting. At the end of the count,
regardless of the status of the SCLx pin, the SCLx pin is
driven low and the Repeated Start condition is complete.
are still high, the SDAx pin is driven low and the BRG is
reloaded and begins counting. At the end of the count,
regardless of the status of the SCLx pin, the SCLx pin is
driven low and the Repeated Start condition is complete.
FIGURE 19-29:
BUS COLLISION DURING A REPEATED START CONDITION (CASE 1)
FIGURE 19-30:
BUS COLLISION DURING REPEATED START CONDITION (CASE 2)
SDAx
SCLx
RSEN
BCLxIF
S
SSPxIF
Sample SDAx when SCLx goes high.
If SDAx = 0, set BCLxIF and release SDAx and SCLx.
If SDAx = 0, set BCLxIF and release SDAx and SCLx.
Cleared in software
‘0’
‘0’
SDAx
SCLx
BCLxIF
RSEN
S
SSPxIF
Interrupt cleared
in software
in software
SCLx goes low before SDAx,
set BCLxIF. Release SDAx and SCLx.
set BCLxIF. Release SDAx and SCLx.
T
BRG
T
BRG
‘0’