
24AA01/24LC01B
DS21711J-page 6 2009 Microchip Technology Inc.
3.0 FUNCTIONAL DESCRIPTION
The 24XX01 supports a bidirectional, 2-wire bus and
data transmission protocol. A device that sends data
onto the bus is defined as transmitter, while defining a
device receiving data as a receiver. The bus has to be
controlled by a master device which generates the
Serial Clock (SCL), controls the bus access and
generates the Start and Stop conditions, while the
24XX01 works as slave. Both master and slave can
operate as transmitter or receiver, but the master
device determines which mode is activated.
4.0 BUS CHARACTERISTICS
The following bus protocol has been defined:
• Data transfer may be initiated only when the bus
is not busy.
• During data transfer, the data line must remain
stab le when ever th e clock lin e is high . Change s in
the data line while the clock line is high will be
interpreted as a Start or Stop condition.
Accordingly, the following bus conditions have been
defined (Figu re 4-1).
4.1 Bus Not Busy (A)
Both data and clock lines remain high.
4.2 Start Data Transfer (B)
A high-to-low transition of the SDA line while the clock
(SCL) is high determines a Start condition. All
commands must be preceded by a Start condition.
4.3 S top Data Transfer (C)
A low-to-high transition of the SDA line while the clock
(SCL) is high determines a Stop condition. All
operations must be ended with a Stop condition.
4.4 Data Valid (D)
The state of the data line represents valid data when,
after a Start condition, the data line is stable for the
duration of the high period of the clock signal.
The data on the line must be changed during the low
period of the clock signal. There is one clock pulse per
bit of data.
Each dat a transf er is initiated w ith a S tart condition an d
terminated with a Stop condition. The number of data
bytes transferred between the Start and Stop
conditions is determined by the master device and is,
theoretically, unlimited (although only the last sixteen
will be st ored wh en do ing a w ri te op era tio n). W hen an
overwrite does occur, it will replace data in a first-in
first- out (FIFO) fashion.
4.5 Acknowledge
Each receiving device, when addressed, is obliged to
generate an acknowledge after the reception of each
byte. Th e mast er device mus t ge nera te a n ex tra c lock
pulse which is associated with this Acknowledge bit.
The device that acknowledges has to pull down the
SDA line d uring th e ackn owledge clock pulse in such a
way that the SDA line is stable-low during the high
period of the acknowledge-related clock pulse. Of
course, setup and hold times must be taken into
account. During reads, a master must signal an end of
data to t he sla ve by not ge nerati ng an Ac knowl edge b it
on the las t by te that has be en c loc ke d ou t of th e sl av e.
In this c as e, the sl ave (24XX01) w ill le av e th e d at a line
high to enable the master to generate the Stop
condition.
FIGURE 4-1: DAT A TRANSFER SEQUENCE ON THE SERIAL BUS
Note: The 24XX01 does not generate any
Acknowledge bits if an internal
programming cycle is in progress.
SCL
SDA
(A) (B) (D) (D) (A)(C)
Start
Condition Address or
Acknowledge
Valid
Data
Allowed
to Change
Stop
Condition