Motorola ColdFire MCF5281 - Manual (Page 462)

I 2 C Interface
24-8 Freescale Semiconductor
Figure 24-7. I
2
C Standard Communication Protocol
24.3.2 Slave Address Transmission
The master sends the slave address in the first byte after the START signal (B). After the seven-bit calling
address, it sends the R/W bit (C), which tells the slave data transfer direction (0 equals write transfer, 1
equals read transfer).
Each slave must have a unique address. An I
2
C master must not transmit its own slave address; it cannot
be master and slave at the same time.
The slave whose address matches that sent by the master pulls I2C_SDA low at the ninth serial clock (D)
to return an acknowledge bit.
24.3.3 Data Transfer
When successful slave addressing is achieved, data transfer can proceed (see E in Figure 24-7 ) on a
byte-by-byte basis in the direction specified by the R/W bit sent by the calling master.
Data can be changed only while I2C_SCL is low and must be held stable while I2C_SCL is high, as
Figure 24-7 shows. I2C_SCL is pulsed once for each data bit, with the msb being sent first. The receiving
device must acknowledge each byte by pulling I2C_SDA low at the ninth clock; therefore, a data byte
transfer takes nine clock pulses. See
Figure 24-8 .
Figure 24-8. Data Transfer
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 9 9
AD7 AD6 AD5 AD4 AD3 AD2 AD1 R/W XXX D7 D6 D5 D4 D3 D2 D1 D0
Calling Address
R/W ACK
Bit
Data Byte
No
ACK
Bit
STOP
Signal
lsb msb lsb msb
START
Signal
A
B
D C
E
F
Interrupt bit set
(Byte complete)
I2C_SCL
I2C_SDA
I2C_ SCL held low while
Interrupt is serviced
1 2 3 4 5 6 7 8 9 5 6 7 8 4 3 2 1
Bit6 Bit4 Bit3 Bit2 Bit1 Bit5 Bit7
Bit0
Bit6 Bit4 Bit3 Bit2 Bit1 Bit5 Bit0 Bit7
START
Signal
ACK from
Receiver
STOP No
ACK Bit
Data Byte Slave Address
R/W
Signal
Interrupt Bit Set
(Byte Complete)
9
I2C_SCL
I2C_SDA
I2C_ SCL held low while
Interrupt is serviced
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3