PIC16F7X7 datasheet(130/276 Pages) MICROCHIP

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PIC16F7X7

DS30498C-page 128

 2004 Microchip Technology Inc.

10.4.17.1

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 10-26).

SCL is sampled low before SDA is asserted low

(Figure 10-27).

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 10-26).

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 10-28). 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 10-26:

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

collision 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.

SDA

SCL

SEN

SDA sampled low before

SDA goes low before the SEN bit is set.

S bit and SSPIF set because

SSP module resets 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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