© 2006 Microchip Technology Inc. DS21204E-page 7
25AA040/25LC040/25C040
3.0 FUNCTIONAL DESCRIPTION
3.1 Principles of Operation
The 25XX040 is a 512 byte Serial EEPROM designed
to interface directly with the Serial Peripheral Interface
(SPI) port of many of today’s popular microcontroller
families, including Microchip’s PIC16C6X/7X micro-
controllers. It may also interface with microcontrollers
that do not have a built-in SPI port by using discrete
I/O lines programmed properly with the software.
The 25XX040 contains an 8-bit instruction register. The
part is accessed via the SI pin, with data being clocked
in on the rising edge of SCK. The CS
pin must be low
and the HOLD
pin must be high for the entire operation.
The WP
pin must be held high to allow writing to the
memory array.
Table 3-1 contains a list of the possible instruction
bytes and format for device operation. The Most
Significant address bit (A8) is located in the instruction
byte. All instructions, addresses, and data are
transferred MSB first, LSB last.
Data is sampled on the first rising edge of SCK after CS
goes low. If the clock line is shared with other periph-
eral devices on the SPI bus, the user can assert the
HOLD input and place the 25XX040 in ‘HOLD’ mode.
After releasing the HOLD
pin, operation will resume
from the point when the HOLD
was asserted.
3.2 Read Sequence
The part is selected by pulling CS low. The 8-bit READ
instruction with the A8 address bit is transmitted to the
25XX040 followed by the lower 8-bit address (A7
through A0). After the correct READ instruction and
address are sent, the data stored in the memory at the
selected address is shifted out on the SO pin. The data
stored in the memory at the next address can be read
sequentially by continuing to provide clock pulses. The
internal Address Pointer is automatically incremented
to the next higher address after each byte of data is
shifted out. When the highest address is reached
(01FFh), the address counter rolls over to address
0000h allowing the read cycle to be continued
indefinitely. The read operation is terminated by raising
the CS
pin (Figure 3-1).
3.3 Write Sequence
Prior to any attempt to write data to the 25XX040, the
write enable latch must be set by issuing the WREN
instruction (Figure 3-4). This is done by setting CS
low
and then clocking out the proper instruction into the
25XX040. After all eight bits of the instruction are
transmitted, the CS
must be brought high to set the
write enable latch. If the write operation is initiated
immediately after the WREN instruction without CS
being brought high, the data will not be written to the
array because the write enable latch will not have been
properly set.
Once the write enable latch is set, the user may
proceed by setting the CS
low, issuing a WRITE
instruction, followed by the address, and then the data
to be written. Keep in mind that the Most Significant
address bit (A8) is included in the instruction byte. Up
to 16 bytes of data can be sent to the 25XX040 before
a write cycle is necessary. The only restriction is that all
of the bytes must reside in the same page. A page
address begins with
XXXX 0000 and ends with XXXX
1111
. If the internal address counter reaches XXXX
1111
and the clock continues, the counter will roll back
to the first address of the page and overwrite any data
in the page that may have been written.
For the data to be actually written to the array, the CS
must be brought high after the least significant bit (D0)
of the n
th
data byte has been clocked in. If CS is
brought high at any other time, the write operation will
not be completed. Refer to Figure 3-2 and Figure 3-3
for more detailed illustrations on the byte write
sequence and the page write sequence respectively.
While the write is in progress, the STATUS register may
be read to check the status of the WIP, WEL, BP1 and
BP0 bits (Figure 3-6). A read attempt of a memory
array location will not be possible during a write cycle.
When the write cycle is completed, the write enable
latch is reset.
TABLE 3-1: INSTRUCTION SET
Instruction Name Instruction Format Description
READ
0000 A8011
Read data from memory array beginning at selected address
WRITE
0000 A8010
Write data to memory array beginning at selected address
WRDI
0000 0100
Reset the write enable latch (disable write operations)
WREN
0000 0110
Set the write enable latch (enable write operations)
RDSR
0000 0101
Read STATUS register
WRSR
0000 0001
Write STATUS register
Note: A
8 is the 9
th
address bit necessary to fully address 512 bytes.
