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PIC16F87X

DS30292C-page 76

2001 Microchip Technology Inc.

FIGURE 9-7:

I2C WAVEFORMS FOR TRANSMISSION (7-BIT ADDRESS)

9.2.2

GENERAL CALL ADDRESS

SUPPORT

The addressing procedure for the I2C bus is such that

the first byte after the START condition usually deter-

mines which device will be the slave addressed by the

master. The exception is the general call address, which

can address all devices. When this address is used, all

devices should, in theory, respond with an acknowledge.

The general call address is one of eight addresses

reserved for specific purposes by the I2C protocol. It

consists of all 0’s with R/W = 0.

The general call address is recognized when the Gen-

eral Call Enable bit (GCEN) is enabled (SSPCON2<7>

is set). Following a START bit detect, 8 bits are shifted

into SSPSR and the address is compared against

SSPADD. It is also compared to the general call

address and fixed in hardware.

If the general call address matches, the SSPSR is

transferred to the SSPBUF, the BF flag is set (eighth

bit), and on the falling edge of the ninth bit (ACK bit),

the SSPIF flag is set.

When the interrupt is serviced, the source for the inter-

rupt can be checked by reading the contents of the

SSPBUF to determine if the address was device spe-

cific, or a general call address.

In 10-bit mode, the SSPADD is required to be updated

for the second half of the address to match, and the UA

bit is set (SSPSTAT<1>). If the general call address is

sampled when GCEN is set, while the slave is config-

ured in 10-bit address mode, then the second half of

the address is not necessary, the UA bit will not be set,

and the slave will begin receiving data after the

Acknowledge (Figure 9-8).

FIGURE 9-8:

SLAVE MODE GENERAL CALL ADDRESS SEQUENCE (7 OR 10-BIT MODE)

SDA

SCL

SSPIF

BF (SSPSTAT<0>)

CKP (SSPCON<4>)

ACK

Not ACK

Transmitting Data

R/W = 1

Receiving Address

5678

Cleared in software

SSPBUF is written in software

From SSP Interrupt

Service Routine

Set bit after writing to SSPBUF

Data in

sampled

SCL held low

while CPU

responds to SSPIF

(the SSPBUF must be written to,

before the CKP bit can be set)

R/W = 0

SDA

SCL

SSPIF

SSPOV

Cleared in software

SSPBUF is read

R/W = 0

ACK

General Call Address

Address is compared to General Call Address

GCEN

Receiving data

ACK

after ACK, set interrupt flag

’0’

’1’

(SSPSTAT<0>)

(SSPCON<6>)

(SSPCON2<7>)

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