8AT24CS128 1152F–SEEPR–7/03
Device
Addressing The 128K EEPROM requires an 8-bit device address word foll owing a st art c ondition to enable
the chip for a read or write operation (refer to Figure 1). The device address word consists of a
mandatory one, zero sequence for the first five most significant bits as shown. This is common
to all 2 -w ire EEPR O M dev ices.
The 128K uses the two device address bits A1, A0 to allow as many as four devices on the
same bus. Th ese bits mus t com pare to their correspondi ng hardwired input pins. The A1 and
A0 pi ns use an intern al proprie tary circui t that biases them to a l ogic low con dition if t he pins
are allowed to float.
The eigh th bit of the devic e address is the rea d/write operatio n select bit. A rea d operation is
initiated if this bit is high and a write operation is initiated if this bit is low.
Upon a compare of the device address, the EEPROM will ou tput a zer o. If a compare is not
made, the device w ill retu rn to a standby state.
DATA SECU RITY: Th e AT 24C S12 8 has a h ardwa re da ta p rote ction sche me th at a llo ws th e
user to write protect the whole memo ry when the WP pin is at VCC.
Write
Operations BY TE WRITE: A write operatio n requires t wo 8-bit dat a word addre sses follo wing the devi ce
address word and acknowledgment. Upon receipt of this address, the EEPROM will again
respond with a zero and then c lock in the first 8-bit data word. Following receipt of the 8-bit
data word, the EEPROM will output a zero. The addressing dev ice, such as a m icrocont roller,
then must terminate the write sequence with a stop condition. At this time the EEPROM enters
an internally-timed wri te cycle, tWR, to th e nonvo lat ile mem ory. All input s ar e disab led du ring
this write cycle and the EEPROM will not respond until the write is complete (refer to Figure 2).
PA GE WRITE: The 128K EEPRO M is capable of 64-byte page writes.
A page write is initiated the same way as a byte write, bu t the microco ntroller does not se nd a
stop condition after the first data word is clocked in. Instead, after the EEPROM acknowledges
receipt of the first data word, the m icrocont roller can transm it up to 63 more data words. Th e
EEPROM will respond with a zero after each data word received. The microcontroller must ter-
minate the page write sequence with a stop condition (refer to Figure 3).
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 locati on. When the w ord ad dre ss, in ternal ly gen er ated, reache s the pa ge bou ndary , th e
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 curre nt page to the
first b yte of the same page.
ACKNOWLEDGE POLLING: Once the internally-timed write cycle has started and the
EEPROM inputs are disabled, ackn owledge polling can be initiated. This involves sending a
start condition followed by t he device address word . The read/write bit is repres entative 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.