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Sheet目录3889

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2001 Microchip Technology Inc.

DS30292C-page 89

PIC16F87X

9.2.18

MULTI -MASTER

COMMUNICATION,

BUS COLLISION, AND

BUS ARBITRATION

Multi-Master mode support is achieved by bus arbitra-

tion. When the master outputs address/data bits onto

the SDA pin, arbitration takes place when the master

outputs a ’1’ on SDA, by letting SDA float high and

another master asserts a ’0’. When the SCL pin floats

high, data should be stable. If the expected data on

SDA is a ’1’ and the data sampled on the SDA pin = ’0’,

a bus collision has taken place. The master will set the

Bus Collision Interrupt Flag, BCLIF and reset the I2C

port to its IDLE state (Figure 9-19).

If a transmit was in progress when the bus collision

occurred, the transmission is halted, the BF flag is

cleared, the SDA and SCL lines are de-asserted, and

the SSPBUF can be written to. When the user services

the bus collision Interrupt Service Routine, and if the

I2C bus is free, the user can resume communication by

asserting a START condition.

If a START, Repeated START, STOP, or Acknowledge

condition was in progress when the bus collision

occurred, the condition is aborted, the SDA and SCL

lines are de-asserted, and the respective control bits in

the SSPCON2 register are cleared. When the user ser-

vices the bus collision Interrupt Service Routine, and if

the I2C bus is free, the user can resume communication

by asserting a START condition.

The master will continue to monitor the SDA and SCL

pins and if a STOP condition occurs, the SSPIF bit will

be set.

A write to the SSPBUF will start the transmission of

data at the first data bit, regardless of where the trans-

mitter left off when the bus collision occurred.

In Multi-Master mode, the interrupt generation on the

detection of START and STOP conditions allows the

determination of when the bus is free. Control of the I2C

bus can be taken when the P bit is set in the SSPSTAT

cleared.

FIGURE 9-19:

BUS COLLISION TIMING FOR TRANSMIT AND ACKNOWLEDGE

SDA

SCL

BCLIF

SDA released

SDA line pulled low

by another source

Sample SDA. While SCL is high,

data doesn’t match what is driven

Bus collision has occurred.

Set bus collision

interrupt

by the master.

by master

Data changes

while SCL = 0

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