10

AT24C128/256

5121B–SEEPR–2/07

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 10 on page 11).

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 11 on page 11).

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 12 on page 11).

Figure 7. Device Address

Figure 8. Byte Write