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PIC16F87XA

DS39582B-page 106

2003 Microchip Technology Inc.

9.4.17.1

Bus Collision During a Start

Condition

During a Start condition, a bus collision occurs if:

SDA or SCL are sampled low at the beginning of

the Start condition (Figure 9-26).

SCL is sampled low before SDA is asserted low

(Figure 9-27).

During a Start condition, both the SDA and the SCL

pins are monitored.

If the SDA pin is already low, or the SCL pin is already

low, then all of the following occur:

the Start condition is aborted,

the BCLIF flag is set and

the MSSP module is reset to its Idle state

(Figure 9-26).

The Start condition begins with the SDA and SCL pins

deasserted. When the SDA pin is sampled high, the

Baud Rate Generator is loaded from SSPADD<6:0>

and counts down to 0. If the SCL pin is sampled low

while SDA 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 SDA pin is sampled low during this count, the

BRG is reset and the SDA line is asserted early

(Figure 9-28). If, however, a ‘1’ is sampled on the SDA

pin, the SDA pin is asserted low at the end of the BRG

count. The Baud Rate Generator is then reloaded and

counts down to 0 and during this time, if the SCL pin is

sampled as ‘0’, a bus collision does not occur. At the

end of the BRG count, the SCL pin is asserted low.

FIGURE 9-26:

BUS COLLISION DURING START CONDITION (SDA 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 SDA before the other.

This condition does not cause a bus colli-

sion because the two masters must be

allowed to arbitrate the first address fol-

lowing the Start condition. If the address is

the same, arbitration must be allowed to

continue into the data portion, Repeated

Start or Stop conditions.

SDA

SCL

SEN

SDA sampled low before

SDA goes low before the SEN bit is set.

S bit and SSPIF set because

SSP module reset into Idle state.

SEN cleared automatically because of bus collision.

S bit and SSPIF set because

Set SEN, enable Start

condition if SDA = 1, SCL = 1

SDA = 0, SCL = 1.

BCLIF

SSPIF

SDA = 0, SCL = 1.

SSPIF and BCLIF are

cleared in software

SSPIF and BCLIF are

cleared in software

Set BCLIF,

Start condition. Set BCLIF.

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