I2C BUS PROTOCOL
The features of the I2C bus protocol are defined as
(1) Data transfer may be initiated only when the bus is
(2) During a data transfer, the data line must remain
stable whenever the clock line is high. Any changes
in the data line while the clock line is high will be
interpreted as a START or STOP condition.
The START Condition precedes all commands to the
device, and is defined as a HIGH to LOW transition of
SDA when SCL is HIGH. The CAT24WC32/64 monitors
the SDA and SCL lines and will not respond until this
condition is met.
A LOW to HIGH transition of SDA when SCL is HIGH
determines the STOP condition. All operations must end
with a STOP condition.
The bus Master begins a transmission by sending a
START condition. The Master sends the address of the
particular slave device it is requesting. The four most
significant bits of the 8-bit slave address are fixed as
1010 (Fig. 5). The next three bits (A2, A1, A0) are the
device address bits; up to eight 32K/64K devices may
to be connected to the same bus. These bits must
compare to the hardwired input pins, A2, A1 and A0. The
last bit of the slave address specifies whether a Read or
Write operation is to be performed. When this bit is set
to 1, a Read operation is selected, and when set to 0, a
Write operation is selected.
After the Master sends a START condition and the slave
address byte, the CAT24WC32/64 monitors the bus and
responds with an acknowledge (on the SDA line) when
its address matches the transmitted slave address. The
CAT24WC32/64 then performs a Read or Write opera-
tion depending on the state of the R/W bit.
After a successful data transfer, each receiving device is
required to generate an acknowledge. The Acknowledg-
ing device pulls down the SDA line during the ninth clock
cycle, signaling that it received the 8 bits of data.
The CAT24WC32/64 responds with an acknowledge
after receiving a START condition and its slave address.
If the device has been selected along with a write
operation, it responds with an acknowledge after receiv-
ing each 8-bit byte.
When the CAT24WC32/64 begins a READ mode it
transmits 8 bits of data, releases the SDA line, and
monitors the line for an acknowledge. Once it receives
this acknowledge, the CAT24WC32/64 will continue to
transmit data. If no acknowledge is sent by the Master,
the device terminates data transmission and waits for a
STOP condition. The master must then issue a stop
condition to return the CAT24WC32/64 to the standby
power mode and place the device in a known state.
Figure 4. Acknowledge Timing
Figure 5. Slave Address Bits
1 0 1 0 A2 A1 A0 R/W
© 2005 by Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
5020 FHD F06
5027 FHD F07
Doc. No. 1039, Rev. G