24LC22A-I Datasheet, PDF (5/24 Pages) Microchip Technology – 2K VESA E-EDID Serial EEPROM

24LC22A
3.0 BIDIRECTIONAL MODE
Before the 24LC22A can be switched into the Bidirec-
tional 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 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 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 excep-
tion that a logic high level is required to enable write
capability. In Bidirectional mode the user has access to
the entire 2K array, whereas in the Transmit-Only mode
the user can only access the first 1K. This mode sup-
ports a two-wire Bidirectional data transmission proto-
col (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 24LC22A acts as
the slave. Both master and slave can operate as trans-
mitter or receiver, but the master device determines
which mode is activated. In the Bidirectional mode, the
24LC22A only responds to commands for device
‘1010 000X’.
FIGURE 3-1:
MODE TRANSITION WITH RECOVERY TO TRANSMIT-ONLY MODE
Transmit-Only
MODE
Bidirectional
TVHZ
Recovery to Transmit-Only mode
SCL
SDA
VCLK count =
VCLK
1 234
(MSB of data in 00h)
Bit 8
127 128
FIGURE 3-2:
SUCCESSFUL MODE TRANSITION TO BIDIRECTIONAL MODE
Transmit-Only
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
© 2007 Microchip Technology Inc.
DS21683B-page 5