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2004 Microchip Technology Inc.

DS30491C-page 219

PIC18F6585/8585/6680/8680

17.4.10

I2C MASTER MODE

TRANSMISSION

Transmission of a data byte, a 7-bit address, or the

other half of a 10-bit address is accomplished by simply

writing a value to the SSPBUF register. This action will

set the Buffer Full flag bit, BF and allow the Baud Rate

Generator to begin counting and start the next trans-

mission. Each bit of address/data will be shifted out

onto the SDA pin after the falling edge of SCL is

asserted (see data hold time specification parameter

#106). SCL is held low for one Baud Rate Generator

rollover count (TBRG). Data should be valid before SCL

is released high (see data setup time specification

parameter #107). When the SCL pin is released high, it

is held that way for TBRG. The data on the SDA pin

must remain stable for that duration and some hold

time after the next falling edge of SCL. After the eighth

bit is shifted out (the falling edge of the eighth clock),

the BF flag is cleared and the master releases SDA.

This allows the slave device being addressed to

respond with an ACK bit during the ninth bit time if an

address match occurred, or if data was received

properly. The status of ACK is written into the ACKDT

bit on the falling edge of the ninth clock. If the master

receives an Acknowledge, the Acknowledge Status bit,

ACKSTAT, is cleared. If not, the bit is set. After the ninth

clock, the SSPIF bit is set and the master clock (Baud

Rate Generator) is suspended until the next data byte

is loaded into the SSPBUF, leaving SCL low and SDA

unchanged (Figure 17-21).

After the write to the SSPBUF, each bit of the address

will be shifted out on the falling edge of SCL until all

seven address bits and the R/W bit are completed. On

the falling edge of the eighth clock, the master will

deassert the SDA pin, allowing the slave to respond

with an Acknowledge. On the falling edge of the ninth

clock, the master will sample the SDA pin to see if the

address was recognized by a slave. The status of the

ACK bit is loaded into the ACKSTAT status bit

(SSPCON2<6>). Following the falling edge of the ninth

clock transmission of the address, the SSPIF is set, the

BF flag is cleared and the Baud Rate Generator is

turned off until another write to the SSPBUF takes

place, holding SCL low and allowing SDA to float.

17.4.10.1

BF Status Flag

In Transmit mode, the BF bit (SSPSTAT<0>) is set

when the CPU writes to SSPBUF and is cleared when

all 8 bits are shifted out.

17.4.10.2

WCOL Status Flag

If the user writes the SSPBUF when a transmit is

already in progress (i.e., SSPSR is still shifting out a

data byte), the WCOL is set and the contents of the

buffer are unchanged (the write doesn’t occur).

WCOL must be cleared in software.

17.4.10.3

ACKSTAT Status Flag

In Transmit mode, the ACKSTAT bit (SSPCON2<6>) is

cleared when the slave has sent an Acknowledge

(ACK = 0) and is set when the slave does not Acknowl-

edge (ACK = 1). A slave sends an Acknowledge when

it has recognized its address (including a general call)

or when the slave has properly received its data.

17.4.11

I2C MASTER MODE RECEPTION

Master mode reception is enabled by programming the

receive enable bit, RCEN (SSPCON2<3>).

The Baud Rate Generator begins counting and on each

rollover, the state of the SCL pin changes (high-to-low/

low-to-high) and data is shifted into the SSPSR. After the

falling edge of the eighth clock, the receive enable flag is

automatically cleared, the contents of the SSPSR are

loaded into the SSPBUF, the BF flag bit is set, the SSPIF

flag bit is set and the Baud Rate Generator is suspended

from counting, holding SCL low. The MSSP is now in Idle

state awaiting the next command. When the buffer is

read by the CPU, the BF flag bit is automatically cleared.

The user can then send an Acknowledge bit at the end

of reception by setting the Acknowledge sequence

enable bit, ACKEN (SSPCON2<4>).

17.4.11.1

BF Status Flag

In receive operation, the BF bit is set when an address

or data byte is loaded into SSPBUF from SSPSR. It is

cleared when the SSPBUF register is read.

17.4.11.2

SSPOV Status Flag

In receive operation, the SSPOV bit is set when 8 bits

are received into the SSPSR and the BF flag bit is

already set from a previous reception.

17.4.11.3

WCOL Status Flag

If the user writes the SSPBUF when a receive is

already in progress (i.e., SSPSR is still shifting in a data

byte), the WCOL bit is set and the contents of the buffer

are unchanged (the write doesn’t occur).

Note:

The MSSP module must be in an Idle state

before the RCEN bit is set or the RCEN bit

will be disregarded.

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