1997-2012 Microchip Technology Inc. DS21230E-page 7
25AA080/25LC080/25C080
3.0 FUNCTIONAL DESCRIPTION
3.1 Principles of Operation
The 25XX080 are 1024 byte Serial EEPROMs
designed to interface directly with the Serial Peripheral
Interface (SPI) port of many of today’s popular micro-
controller families, including Microchip’s PIC16C6X/7X
microcontrollers. It may also interface with microcon-
trollers that do not have a built-in SPI port by using
discrete I/O lines programmed properly with the
software.
The 25XX080 contains an 8-bit instruction register. The
device 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. 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 25XX080 in ‘HOLD’ mode.
After releasing the HOLD
pin, operation will resume
from the point when the HOLD
was asserted.
3.2 Read Sequence
The device is selected by pulling CS low. The 8-bit READ
instruction is transmitted to the 25XX080 followed by
the 16-bit address, with the six MSBs of the address
being "don’t care" bits. 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 incre-
mented to the next higher address after each byte of
data is shifted out. When the highest address is reached
(03FFh), the address counter rolls over to address
0000h allowing the read cycle to be continued indefi-
nitely. 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 25XX080, 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
25XX080. After all eight bits of the instruction are trans-
mitted, the CS
must be brought high to set the write
enable latch. If the write operation is initiated immedi-
ately 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 instruc-
tion, followed by the 16-bit address, with the six MSBs
of the address being “don’t care” bits, and then the data
to be written. Up to 16 bytes of data can be sent to the
25XX080 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 xxxx
xxxx
0000 and ends with xxxx xxxx xxxx 1111.
If the internal address counter reaches
xxxx xxxx
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 WPEN, 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 0011 Read data from memory array beginning at selected address
WRITE 0000 0010 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
