PIC18F44J10 datasheet(190/368 Pages) MICROCHIP

190 / 368 page

PIC18F45J10 FAMILY

DS39682E-page 188

16.4.17.1

Bus Collision During a Start

Condition

During a Start condition, a bus collision occurs if:

SDAx or SCLx are sampled low at the beginning

of the Start condition (Figure 16-26).

SCLx is sampled low before SDAx is asserted

low (Figure 16-27).

During a Start condition, both the SDAx and the SCLx

pins are monitored.

If the SDAx pin is already low, or the SCLx pin is

already low, then all of the following occur:

• the Start condition is aborted;

• the BCLxIF flag is set; and

• the MSSP module is reset to its Idle state

(Figure 16-26).

The Start condition begins with the SDAx and SCLx

pins deasserted. When the SDAx pin is sampled high,

the

Baud

Rate

Generator

loaded

from

SSPxADD<6:0> and counts down to ‘0’. If the SCLx pin

is sampled low while SDAx is high, a bus collision

occurs, because it is assumed that another master is

attempting to drive a data ‘1’ during the Start condition.

If the SDAx pin is sampled low during this count, the

BRG is reset and the SDAx line is asserted early

(Figure 16-28). If, however, a ‘1’ is sampled on the

SDAx pin, the SDAx pin is asserted low at the end of

the BRG count. The Baud Rate Generator is then

reloaded and counts down to 0. If the SCLx pin is

sampled as ‘0’ during this time, a bus collision does not

occur. At the end of the BRG count, the SCLx pin is

asserted low.

FIGURE 16-26:

BUS COLLISION DURING START CONDITION (SDAx ONLY)

Note:

The reason that bus collision is not a factor

during a Start condition is that no two bus

masters can assert a Start condition at the

exact same time. Therefore, one master

will always assert SDAx before the other.

This condition does not cause a bus colli-

sion because the two masters must be

allowed to arbitrate the first address

following the Start condition. If the address

is the same, arbitration must be allowed to

continue into the data portion, Repeated

Start or Stop conditions.

SDAx

SCLx

SEN

SDAx sampled low before

SDAx goes low before the SEN bit is set.

S bit and SSPxIF set because

MSSP module reset into Idle state.

SEN cleared automatically because of bus collision.

S bit and SSPxIF set because

Set SEN, enable Start

condition if SDAx = 1, SCLx = 1

SDAx = 0, SCLx = 1.

BCLxIF

SSPxIF

SDAx = 0, SCLx = 1.

SSPxIF and BCLxIF are

cleared in software

SSPxIF and BCLxIF are

cleared in software

Set BCLxIF,

Start condition. Set BCLxIF.

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