PIC18F242 datasheet(164/332 Pages) MICROCHIP

164 / 332 page

PIC18FXX2

DS39564C-page 162

15.4.17.2

Bus Collision During a Repeated

START Condition

During a Repeated START condition, a bus collision

occurs if:

A low level is sampled on SDA when SCL goes

from low level to high level.

SCL goes low before SDA is asserted low, indi-

cating that another master is attempting to

transmit 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

attempting

transmit

data

’0’,

Figure 15-29). 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.

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 15-30.

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

complete.

FIGURE 15-29:

BUS COLLISION DURING A REPEATED START CONDITION (CASE 1)

FIGURE 15-30:

BUS COLLISION DURING REPEATED START CONDITION (CASE 2)

SDA

SCL

RSEN

BCLIF

SSPIF

Sample SDA when SCL goes high.

If SDA = 0, set BCLIF and release SDA and SCL.

Cleared in software

'0'

SDA

SCL

BCLIF

RSEN

SSPIF

Interrupt cleared

in software

SCL goes low before SDA,

Set BCLIF. Release SDA and SCL.

TBRG

'0'

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