PIC16F872SLASHSP datasheet(81/160 Pages) MICROCHIP

81 / 160 page

Preliminary

DS30221A-page 81

PIC16F872

9.2.18.16 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 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)

FIGURE 9-24: 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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