24LCS21A_05 Datasheet, PDF (5/20 Pages) Microchip Technology – 1K 2.5V Dual Mode I2C™ Serial EEPROM

24LCS21A
3.0 BIDIRECTIONAL MODE
Before the 24LCS21A can be switched into the
Bidirectional mode (Figure 3-1), it must enter the Tran-
sition 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 con-
trol byte, ‘1010 000X’ on the I²C™ bus, and starts to
count pulses on VCLK. Any high-to-low transition 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 con-
tents starting with the Most Significant bit in address
00h. However, if it detects the control byte on the I²C™
bus, (Figure 3-2) it will switch to the Bidirectional mode.
Once the device has made the transition to the Bidirec-
tional 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 (I²C™). 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 24LCS21A 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 24LCS21A only responds
to commands for device ‘1010 000X’.
FIGURE 3-1:
MODE TRANSITION WITH RECOVERY TO TRANSMIT-ONLY MODE
Mode
Transmit-
Only
SCL
Bidirectional
TVHZ
SDA
VCLK count =
VCLK
1234
Recovery to Transmit-Only mode
127 128
(MSB of data in 00h)
Bit8
FIGURE 3-2:
SUCCESSFUL MODE TRANSITION TO BIDIRECTIONAL MODE
Transmit-
Only Mode
MODE
SCL
Bidirectional
Transition mode with possibility to return to Transmit-Only mode permanently
SDA
VCLK count = 1 2
VCLK
S 1 0 1 0 0 0 0 0 ACK
n0
n < 128
© 2005 Microchip Technology Inc.
DS21161G-page 5