PIC18F2331/2431/4331/4431
An SSP interrupt is generated for each data transfer
byte. Flag bit, SSPIF, must be cleared in software and
the SSPSTAT register is used to determine the status
of the byte. Flag bit, SSPIF, is set on the falling edge of
the ninth clock pulse.
19.3.1.3
Transmission
When the R/W bit of the incoming address byte is set
and an address match occurs, the R/W bit of the
SSPSTAT register is set. The received address is
loaded into the SSPBUF register. The ACK pulse will
be sent on the ninth bit and pin, SCK/SCL, is held low.
The transmit data must be loaded into the SSPBUF
register, which also loads the SSPSR register. Then,
pin, SCK/SCL, should be enabled by setting bit, CKP
(SSPCON<4>). The master must monitor the SCL pin
prior to asserting another clock pulse. The slave
devices may be holding off the master by stretching the
clock. The eight data bits are shifted out on the falling
edge of the SCL input. This ensures that the SDA signal
is valid during the SCL high time (Figure 19-7).
As a slave-transmitter, the ACK pulse from the master-
receiver is latched on the rising edge of the ninth SCL
input pulse. If the SDA line was high (not ACK), then the
data transfer is complete. When the ACK is latched by
the slave, the slave logic is reset and the slave then
monitors for another occurrence of the Start bit. If the
SDA line was low (ACK), the transmit data must be
loaded into the SSPBUF register, which also loads the
SSPSR register. Then pin, SCK/SCL, should be enabled
by setting bit CKP.
FIGURE 19-7:
I2C™ WAVEFORMS FOR TRANSMISSION (7-BIT ADDRESS)
Receiving Address
R/W = 1
ACK
Transmitting Data
ACK
9
SDA
A7 A6 A5 A4 A3 A2 A1
D7 D6 D5 D4 D3 D2 D1 D0
SCL
S
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
P
SCL held low
while CPU
responds to SSPIF
Data in
sampled
Cleared in software
SSPIF (PIR1<3>)
BF (SSPSTAT<0>)
CKP (SSPCON<4>)
From SSP Interrupt
Service Routine
SSPBUF is written in software
Set bit after writing to SSPBUF
(SSPBUF must be written to
before the CKP bit can be set)
2010 Microchip Technology Inc.
DS39616D-page 215
MICROCHIP [ MICROCHIP ]
