MCP19124 datasheet(187/236 Pages) MICROCHIP

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 2016 Microchip Technology Inc.

DS20005619A-page 187

MCP19124/5

28.5.13.1

Bus Collision During a Start

Condition

During a Start condition, a bus collision occurs if:

a) SDA or SCL are sampled low at the beginning of

the Start condition (Figure 28-25).

b) SCL is sampled low before SDA is asserted low

(Figure 28-26).

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, all of the following occur:

• The Start condition is aborted.

• The BCLIF flag is set.

• The MSSP module is reset to its Idle state

(Figure 28-25).

The Start condition begins with the SDA and SCL pins

deasserted. When the SDA pin is sampled high, the

Baud Rate Generator is loaded and counts down. 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 28-27). 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 zero; if the SCL pin is sampled as ‘0’

during this time, a bus collision does not occur. At the

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

FIGURE 28-25:

BUS COLLISION DURING A START CONDITION (SDA ONLY)

Note:

The reason why 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

Stop

conditions.

SDA

SCL

SEN

BCLIF

SSPIF

SDA goes low before the SEN bit is set.

Set BCLIF,

S bit and SSPIF set because

SDA = 0, SCL = 1.

Set SEN, enable Start condition

if SDA = 1, SCL = 1

SEN cleared automatically because of bus

collision.

SSP module reset into Idle state.

SDA sampled low before Start condition.

Set BCLIF. S bit and SSPIF set because

SDA = 0, SCL = 1.

SSPIF and BCLIF

are cleared by software

SSPIF and BCLIF

are cleared by software

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