PIC16 datasheet(224/418 Pages) MICROCHIP

224 / 418 page

PIC16(L)F1508/9

DS40001609C-page 224

 2011-2013 Microchip Technology Inc.

21.6.13.1

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 21-33).

SCL is sampled low before SDAx is asserted

low (Figure 21-34).

During a Start condition, both the SDAx 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 BCL1IF flag is set and

• the MSSP module is reset to its Idle state

(Figure 21-33).

The Start condition begins with the SDAx and SCLx

pins deasserted. When the SDAx pin is sampled high,

the Baud Rate Generator is loaded and counts down. 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 21-35). 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 21-33:

BUS COLLISION DURING START CONDITION (SDAX ONLY)

Note:

The reason that bus collision is not a fac-

tor 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

collision 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.

SDAx

SCLx

SEN

SDAx sampled low before

SDAx goes low before the SEN bit is set.

S bit and SSPxIF set because

SSP 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 by software

SSPxIF and BCLxIF are

cleared by software

Set BCLxIF,

Start condition. Set BCLxIF.

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