© 2008 Microchip Technology Inc. DS21073K-page 11
24AA65/24LC65/24C65
7.0 PAGE CACHE AND ARRAY
MAPPING
The cache is a 64-byte (8 pages x 8 bytes) FIFO buffer.
The cache allows the loading of up to 64 bytes of data
before the write cycle is actually begun, effectively
providing a 64-byte burst write at the maximum bus
rate. Whenever a Write command is initiated, the cache
starts loading and will continue to load until a Stop bit is
received to start the internal write cycle. The total
length of the write cycle will depend on how many
pages are loaded into the cache before the Stop bit is
given. Maximum cycle time for each page is 5 ms. Even
if a page is only partially loaded, it will still require the
same cycle time as a full page. If more than 64 bytes of
data are loaded before the Stop bit is given, the
Address Pointer will ‘wrap around’ to the beginning of
cache page 0 and existing bytes in the cache will be
overwritten. The device will not respond to any
commands while the write cycle is in progress.
7.1 Cache Write Starting at a Page
Boundary
If a Write command begins at a page boundary
(address bits A2, A1 and A0 are zero), then all data
loaded into the cache will be written to the array in
sequential addresses. This includes writing across a
4K block boundary. In the example shown below,
(Figure 8-2) a Write command is initiated starting at
byte 0 of page 3 with a fully loaded cache (64 bytes).
The first byte in the cache is written to byte 0 of page 3
(of the array), with the remaining pages in the cache
written to sequential pages in the array. A write cycle is
executed after each page is written. Since the write
begins at page 3 and 8 pages are loaded into the
cache, the last 3 pages of the cache are written to the
next row in the array.
7.2 Cache Write Starting at a
Non-Page Boundary
When a Write command is initiated that does not begin
at a page boundary (i.e., address bits A2, A1 and A0
are not all zero), it is important to note how the data is
loaded into the cache, and how the data in the cache is
written to the array. When a Write command begins, the
first byte loaded into the cache is always loaded into
page 0. The byte within page 0 of the cache where the
load begins is determined by the three Least Significant
Address bits (A2, A1, A0) that were sent as part of the
Write command. If the Write command does not start at
byte 0 of a page and the cache is fully loaded, then the
last byte(s) loaded into the cache will roll around to
page 0 of the cache and fill the remaining empty bytes.
If more than 64 bytes of data are loaded into the cache,
data already loaded will be overwritten. In the example
shown in Figure 8-3, a Write command has been
initiated starting at byte 2 of page 3 in the array with a
fully loaded cache of 64 bytes. Since the cache started
loading at byte 2, the last two bytes loaded into the
cache will ‘roll over' and be loaded into the first two
bytes of page 0 (of the cache). When the Stop bit is
sent, page 0 of the cache is written to page 3 of the
array. The remaining pages in the cache are then
loaded sequentially to the array. A write cycle is
executed after each page is written. If a partially loaded
page in the cache remains when the Stop bit is sent,
only the bytes that have been loaded will be written to
the array.
7.3 Power Management
The design incorporates a power Standby mode when
not in use and automatically powers off after the normal
termination of any operation when a Stop bit is received
and all internal functions are complete. This includes
any error conditions (i.e., not receiving an Acknowl-
edge or Stop condition per the two-wire bus specifica-
tion). The device also incorporates V
DD monitor
circuitry to prevent inadvertent writes (data corruption)
during low voltage conditions. The VDD monitor circuitry
is powered off when the device is in Standby mode in
order to further reduce power consumption.
8.0 PIN DESCRIPTIONS
8.1 A0, A1, A2 Chip Address Inputs
The A0..A2 inputs are used by the 24XX65 for multiple
device operation and conform to the two-wire bus
standard. The levels applied to these pins define the
address block occupied by the device in the address
map. A particular device is selected by transmitting the
corresponding bits (A2, A1, A0) in the control byte
(Figure 3-2 and Figure 8-1).
8.2 SDA Serial Address/Data Input/
Output
This is a bidirectional pin used to transfer addresses
and data into and data out of the device. It is an open
drain terminal, therefore the SDA bus requires a pull-up
resistor to V
CC (typical 10 KΩ for 100 kHz, 2 KΩ for 400
kHz).
For normal data transfer SDA is allowed to change only
during SCL low. Changes during SCL high are
reserved for indicating the Start and Stop conditions.
8.3 SCL Serial Clock
This input is used to synchronize the data transfer from
and to the device.
