9
AT24C256B
0670O–SEEPR–7/04
The data word address lower 6 bits are internally incremented following the receipt of each
data word. The higher data word address bits are not incremented, retaining the memory page
row location. When the word address, internally generated, reaches the page boundary, the
following byte is placed at the beginning of the same page. If more than 64 data words are
transmitted to the EEPROM, the data word address will “roll over” and previous data will be
overwritten. The address “roll over” during write is from the last byte of the current page to the
first byte of the same page.
ACKNOWLEDGE POLLING: Once the internally-timed write cycle has started and the
EEPROM inputs are disabled, acknowledge polling can be initiated. This involves sending a
start condition followed by the device address word. The read/write bit is representative of the
operation desired. Only if the internal write cycle has completed will the EEPROM respond
with a zero, allowing the read or write sequence to continue.
Read
Operations
Read operations are initiated the same way as write operations with the exception that the
read/write select bit in the device address word is set to one. There are three read operations:
current address read, random address read and sequential read.
CURRENT ADDRESS READ: The internal data word address counter maintains the last
address accessed during the last read or write operation, incremented by one. This address
stays valid between operations as long as the chip power is maintained. The address “roll
over” during read is from the last byte of the last memory page, to the first byte of the first
page.
Once the device address with the read/write select bit set to one is clocked in and acknowl-
edged by the EEPROM, the current address data word is serially clocked out. The
microcontroller does not respond with an input zero but does generate a following stop condi-
tion (see Figure 9 on page 10).
RANDOM READ: A random read requires a “dummy” byte write sequence to load in the data
word address. Once the device address word and data word address are clocked in and
acknowledged by the EEPROM, the microcontroller must generate another start condition.
The microcontroller now initiates a current address read by sending a device address with the
read/write select bit high. The EEPROM acknowledges the device address and serially clocks
out the data word. The microcontroller does not respond with a zero but does generate a fol-
lowing stop condition (see Figure 10 on page 10).
SEQUENTIAL READ: Sequential reads are initiated by either a current address read or a ran-
dom address read. After the microcontroller receives a data word, it responds with an
acknowledge. As long as the EEPROM receives an acknowledge, it will continue to increment
the data word address and serially clock out sequential data words. When the memory
address limit is reached, the data word address will “roll over” and the sequential read will con-
tinue. The sequential read operation is terminated when the microcontroller does not respond
with a zero but does generate a following stop condition (see Figure 11 on page 11).
Figure 6. Device Address
MSB
1 0 1 0 A2 A1 A0 R/W
LSB