24LC21A
3.0
BIDIRECTIONAL MODE
Before the 24LC21A can be switched into the
Bidirectional mode (Figure 3-1), it must enter the
Transition mode, which is done by applying a valid
high-to-low transition on the Bidirectional mode clock
(SCL). As soon it enters the Transition mode, it looks
for a control byte
‘1010 000X’
on the I
2
C™ bus, and
starts to count pulses on VCLK. Any high-to-low transi-
tion on the SCL line will reset the count. If it sees a
pulse count of 128 on VCLK while the SCL line is idle,
it will revert back to the Transmit-Only mode, and
transmit its contents starting with the Most Significant
bit in address 00h. However, if it detects the control
byte on the I
2
C™ bus, (Figure 3-2) it will switch to the
in the Bidirectional mode. Once the device has made
the transition to the Bidirectional mode, the only way to
switch the device back to the Transmit-Only mode is to
remove power from the device. The mode transition
process is shown in detail in Figure 3-3.
Once the device has switched into the Bidirectional
mode, the VCLK input is disregarded, with the
exception that a logic high level is required to enable
write capability. This mode supports a two-wire
Bidirectional data transmission protocol (I
2
C™). In this
protocol, a device that sends data on the bus is defined
to be the transmitter, and a device that receives data
from the bus is defined to be the receiver. The bus must
be controlled by a master device that generates the
Bidirectional mode clock (SCL), controls access to the
bus and generates the Start and Stop conditions, while
the 24LC21A acts as the slave. Both master and slave
can operate as transmitter or receiver, but the master
device determines which mode is activated. In the
Bidirectional mode, the 24LC21A only responds to
commands for device
‘1010 000X’.
FIGURE 3-1:
MODE
SCL
MODE TRANSITION WITH RECOVERY TO TRANSMIT-ONLY MODE
Transmit-Only
Bidirectional
TVHZ
Recovery to Transmit-Only mode
(MSB of data in 00h)
SDA
VCLK count =
VCLK
1
2
3
4
127 128
Bit 8
FIGURE 3-2:
SUCCESSFUL MODE TRANSITION TO BIDIRECTIONAL MODE
Transition mode with possibility to return to Transmit-Only mode
Bidirectional
permanently
Transmit-Only
MODE
SCL
SDA
VCLK count =
VCLK
1
2
n
S
1
0
0
1
0
0
0
0
0
ACK
n < 128
©
2008 Microchip Technology Inc.
DS21160G-page 5
MICROCHIP [ MICROCHIP TECHNOLOGY ]
