Serial EEPROM vs. Parallel:-
SERIAL EEPROMS
The main feature that makes a device a “Serial” and sets it apart from parallel
devices is, as its name implies, the ability to communicate through
a serial interface. This
ability has numerous benefits. First,
serial communication
is accomplished with a minimum number of I/O’s.
Serial EEPROMs require only two to four lines (depending
on the hardware and software protocol) for complete
communication; memory addressing, data input and
output, and device control. Thus, the hardware interface
requirements for Serial EEPROMs are kept at a minimum.
The most common Serial EEPROMs in use today
are devices that utilize a 2-wire protocol.
Another benefit of serial communication is package size.
Ranging from densities of 256 to 16K bits, most Serial
EEPROMs today are available in space-saving 8 pin
PDIP and 150mil wide SOIC packaging. This obviously
is very beneficial for applications where product size and
weight is a key design factor. The final benefit is low
current consumption. Due to a limited number of I/O
ports and therefore on-chip support requirements, operating
currents for Serial EEPROMs are usually well
below 3 milliamperes.
Other features of Serial EEPROMs include: 1) Byte
programmability—The ability to erase and program one
byte at a time without affecting the contents of the other
memory locations in the array; 2) Clock rates of up to
6MHz—2-wire devices are rated at 100KHz and 400KHz
per the standard I2C protocol, while 3-wire devices can
be operated at 6M Hz rates; 3) Low voltage operation—
Microchip has introduced a family of devices that operate,
both read and write, down to 1.8V. This family
complements other 2V and 2.5V low voltage Serial
EEPROM families available.
PARALLEL NON-VOLATILE
MEMORIES
There are a number of memory devices that fall into this
category. The major ones include Parallel EEPROMs,
Flash memory products, EPROMs, and SRAMs with
battery back-up.
The main common feature of all of these devices is that
communication with the device is done through a parallel
interface, which results in a high system clock rate.
Each type of device has separate data, address and
control lines. Thus pin counts are in the 24 to 40 pin
range. This also results in relatively large and costly
packages and large footprints, even with the most advanced
surface mount packages like TSOP. SRAMs
with on-board batteries require DIP package heights
that are significantly higher than those of standard DIP
packages, adding to its package size and cost disadvantage.
Parallel EEPROM and battery backed-up SRAMs are
the only two of the four major types of parallel nonvolatile
memories that have the capability to erase and
program one byte at a time. EPROM and Flash devices
require the whole array or at least large sectors to be
erased prior to reprogramming.
SERIAL VERSUS PARALLEL
Serial EEPROMs have five major advantages over
parallel non-volatile memories.
1) Lower Current Consumption - The maximum operating
current (at 5 volts operating voltage) of a 16K
serial EEPROM device is approximately an order of
magnitude less than that of an equivalent density
parallel EEPROM. Operating currents for 16K
Serials are specified at 3mA, while 16K parallel
devices are specified at 30mA and above. This
relationship will continue as 64K serial devices are
introduced. Since power consumption is directly
proportional to current consumption, the lower the
current the lower the power consumption.
2) Lower Voltage - Serial EEPROMs have been available
in single supply low voltage options for some
time. As mentioned above, Microchip has low
voltage Serial EEPROMs that operate down to
1.8V, as well as other low voltage Serials that
function down to 2.0V or 2.5V volts. 3V EPROMs
Serial EEPROM Solutions vs. Parallel Solutions
Serial EEPROM Solutions vs. Parallel Solutions
and parallel EEPROMs and single voltage 5V flash
devices are just being introduced to the market.
(Most flash devices on the market today require
12V for programming in addition to the 5 volts
required for normal operation). Low voltage operation
also has a positive effect on power consumption.
A reduction in the operating voltage from 5
volts to 1.8 volts will result in a power consumption
reduction of almost 90% and almost a 65% reduction
in power if the operating voltage is reduced from
3V to 1.8V.
3) Programmability - Neither currently available Flash
devices nor EPROMs have the ability to program
one byte at a time. Erasing is an array or sector
function. Therefore, whenever one byte needs to be
reprogrammed the entire array or sector must be
reprogrammed. This procedure takes a relatively
long amount of time to complete, time which may
not be available, as is the case when storing critical
parameters or data during inadvertent and unexpected
system power loss. This procedure also
requires software overhead to manage the retrieval
and reprogramming operation.
4) Physical Size - Again, when comparing a 16K Serial
EEPROM to a 16K parallel device the serial has a
significant advantage. The area of the 150mil 8 pin
SOIC footprint is less than 50K square mils. This
compares to an area of more than 250K square mils
for a 24 pin SOIC and almost 800K square mils for
a 24 pin 500mil DIP package footprint.
5) I/O Requirements - Serials only require 2 to 4 input
or output lines for complete communications. Most
parallel devices require at least 22 lines, depending
on the memory density. This results in increased
microcontroller/microprocessor overhead and additional
real estate to accommodate the numerous
hardware lines.
The advantages that parallel devices currently have
over serial EEPROMs is memory density and AC performance.
However, in most microcontroller based applications
for which Serial EEPROMs are intended, high
density and AC are not the most critical design issues or
most needed product features.
The key benefits of Serial EEPROM solutions as a result
of the advantages outlined above, are reduced system
costs, enhanced feature sets, and improved system
performance. System size and weight is reduced and
power sourcing requirements are kept at a minimum.
The following graph compares some of the main attributes
of a 16K Serial EEPROM device to a 16K
Parallel device.
8
Serial EEPROM Solutions vs. Parallel Solutions
USES AND APPLICATIONS OF
SERIAL EEPROMS
Uses of Serial EEPROMs
The days of simply being a DIP switch replacement for
Serial EEPROMs is over. Here is a list of the functions
that Serial EEPROMs perform in a variety of computer,
industrial, telecommunication, automotive and consumer
applications:
1) Memory storage of channel selectors or analog
controls (volume, tone, etc.)
2) Power down storage and retrieval of events such as
fault detection or error diagnostics
3) Electronic real time event or maintenance log such
as page counting
4) Configuration storage
5) Last number redial and speed dial storage
6) User in-circuit look-up tables
Serial EEPROM Applications
Serial EEPROMs have found homes in hundreds of
embedded control applications in all major application
markets. The following list demonstrates the number
and variety of applications for serial EEPROMs.
Market Applications
CONSUMER TV tuners, VCRs, CD players, cameras,
radios, and remote controls
COMPUTER/OA Printers, copiers, PCs, palmtop and
portable computers, disk drives and
organizers
INDUSTRIAL Bar code readers, point-of-sale terminals,
smart cards, lock boxes,
garage door openers, test measurement
equipment and medical equipment
TELECOMM Cellular, cordless and full feature
phones, faxes, modems, pagers, and
satellite receivers
AUTOMOTIVE Air bags, anti-lock brakes, odometers,
radios and keyless entry
Using Serial EEPROMs for critical data and configuration
storage has only recently become a reality. The
current offerings of 2- and 3-wire serial devices offers the
systems designer interesting alternatives to the standard
parallel EEPROM devices. The Serial EEPROM is
basically a standard EEPROM array without the normal
parallel data and address I/O. These functions are
handled via serial I/O ports coupled with internal selftimed
state machines. Not only will the serial device
Serial EEPROM Solutions vs. Parallel Solutions
save power, board space, and cost, but they also offer
the advantage of fewer I/O and consequently power in
the embedded microcontroller because less I/O are
needed to control the same functions. A typical embedded
application is shown in Figure A, depicting a controller
and several functions used in a personal communications
device, such as a mobile or portable phone. The
EEPROM stores speed dial and last number redial
numbers, credit card numbers, ID numbers, and configuration
parameters.
Figure B shows these same functions using a controller
with fewer I/O and a Serial EEPROM. There is no loss of
functionality but a significant savings in current, board
space, I/O pads, and cost. The serial solution employs
8 to 16 less I/O on the microcontroller, freeing up much
needed functionality, and possibly allowing for a much
smaller device package and downsized circuit boards.
SUMMARY
Serial EEPROMs are ideal cost effective solutions to all
non-volatile memory embedded control applications
that require: 1) A small footprint space saving format;
2) The ability and ease of programming one byte at a
time; 3) Low current consumption and low operating
voltage; 4) Low microcontroller overhead and support;
and, 5) The best price performance non-volatile memory
solution available.
Their size, ease of programmability, low power consumption,
and low cost make Serial EEPROMs extremely
suitable for all the fast growing handheld and
portable battery powered computer, personal communications,
medical and industrial markets.
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