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PIC16F87X

DS30292C-page 92

2001 Microchip Technology Inc.

9.2.18.2

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 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

complete (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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