HT46RU25/HT46CU25
Start Signal
Acknowledge Bit
The START signal is generated only by the master de-
vice. The other device in the bus must detect the START
signal to set the I2C Bus busy bit (HBB). The START sig-
nal is SDA line from high to low, when SCL is high.
One of the slave device generates an acknowledge signal,
when the slave address is matched. The master device
can check this acknowledge bit to know if the slave device
accepts the calling address. If no acknowledge bit, the
master must send a STOP bit and end the communication.
When the I2C Bus status register bit 6 HAAS is high, it
means the address is matched, so the slave must check
the SRW as a transmitter (set HTX) to ²1² or as a re-
ceiver (clear HTX) to ²0².
S
C
L
S
D
A
Start Bit
S
S
C
D
L
Slave Address
A
The master must select a device for transferring the
data by sending the slave device address after the
START signal. All device in the I2C Bus will receive the
I2C Bus slave address (7 bits) to compare with its own
slave address (7 bits). If the slave address is matched,
the slave device will generate an interrupt and save the
subsequent bit (8th bit) to the SRW bit and sends an ac-
knowledge bit (low level) to the 9th bit. The slave device
also sets the status flag (HAAS), when the slave ad-
dress is matched.
Stop Bit
Data Byte
The data is 8 bits and is sent after the slave device has
acknowledged the slave address. The first bit is MSB
and the 8th bit is LSB. The receiver sends the acknowl-
edge signal (²0²) and continues to receive the next one
byte data. If the transmitter checks and there¢s no ac-
knowledge signal, then it release the SDA line, and the
master sends a STOP signal to release the I2C Bus. The
data is stored in the HDR register. The transmitter must
write data to the HDR before transmitting data and the
receiver must read data from the HDR after receiving
data.
In interrupt subroutine, check the HAAS bit to know
whether the I2C Bus interrupt comes from a slave ad-
dress that is matched or a data byte transfer is com-
pleted. When the slave address is matched, the device
must be in transmit mode or receive mode and write
data to HDR or dummy read from HDR to release the
SCL line.
S
S
C
L
D
A
SRW Bit
The SRW bit means that the master device wants to
read from or write to the I2C Bus. The slave device
check this bit to understand itself if it is a transmitter or a
receiver. The SRW bit is set to ²1² means that the mas-
ter wants to read data from the I2C Bus, so the slave de-
vice must write data to a bus as a transmitter. The SRW
is cleared to ²0² means that the master wants to write
data to the I2C Bus, so the slave device must read data
from the I2C Bus as a receiver.
S
t
o
p
b
i
t
S
t
a
r
t
b
i
t
D
a
t
a
D
a
t
a
s
t
a
b
l
e
a
l
l
o
w
c
h
a
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e
Data Timing Diagram
Receive Acknowledge Bit
When the receiver wants to continue to receive the next
data byte, it generates an acknowledge bit (TXAK) at
the 9th clock. The transmitter checks the acknowledge
bit (RXAK) to continue to write data to the I2C Bus or
change to receive mode and a dummy reads the HDR
register to release the SDA line and the master sends
the STOP signal.
Rev. 1.30
31
March 9, 2007
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
