PIC17C752-08SLASHCL datasheet(171/328 Pages) MICROCHIP

171 / 328 page

DS30289A-page 171

PIC17C7XX

15.2.18.2 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 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 mas-

ter 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 15-41)

FIGURE 15-41: BUS COLLISION DURING A REPEATED START CONDITION (CASE 1)

FIGURE 15-42: 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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