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
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.