PIC16LC771 datasheet(97/224 Pages) MICROCHIP

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DS41120C-page 97

PIC16C717/770/771

9.2.17.2

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 trans-

mit a data ’1’.

When the master module 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, then 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, then

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 9-27).

FIGURE 9-27:

BUS COLLISION DURING A REPEATED START CONDITION (CASE 1)

FIGURE 9-28:

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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