Using the microchip endurance predictive software

Using Endurance Predictive Software
Using the Microchip Endurance Predictive Software
INTRODUCTION
TOTAL ENDURANCE PREDICTIVE
SOFTWARE
Endurance, as it applies to non-volatile memory, refersto the number of times an individual memory cell can be The predictive software described here originally was erased and/or written (some architectures do not erase being developed as a tool for determining endurance before every write). Advances in process technology levels of Microchip non-volatile devices. Upon seeing have made it possible to increase these limits and for the potential of the software as a design aid, it was Microchip to offer new concepts - Total Endurance™ and decided to make a version that could be purchased by a split architectural design for variable endurance. These the engineering community. The benefit gained from this concepts lead to more reliable products with more bits software is the ability to predict endurance capability of per dice, such as the 24C32 and 24C65.
Microchip’s EEPROM devices under various operatingconditions. Prior to this tool becoming available, the only TOTAL ENDURANCE ™
way to assemble this type of data would be to doextensive life testing in the target system. It should be When defining endurance, we need to look at a few noted that this predictive model applies only to Microchip common definitions and possible misconceptions. En- Technology Inc. non-volatile devices.
durance with respect to EEPROMs is defined in numberof Erase/Write (E/W) Cycles and is the most common The program uses an iterative statistical model devel- rating referred to when discussing or specifying endur- oped by Microchip Technology Inc. physicists. The ance. E/W ratings are based on the environmental and model was first used in a DOS-based text program as a operating conditions of voltage, temperature, cycling proof of concept and for developing the exhaustive mode and rate (for each byte in the application not on the database needed for such a tool (included on the pro- number of opcodes or control byte commands) and is gram disk as enddos.exe). This model was then im- never based on any read functions whether they be a ported to a Windows™-based software package with full data read or configuration read. If a part is rated at 100K GUI capabilities and all the normal cut, paste, print, E/W cycles, then each individual byte can be erased and viewing properties. The model actually operates as a written 100,000 times. This is probably the most com- mathematical function which is called from within the mon misinterpretation made by system designers. En- Windows Visual Basic shell and is passed all of the durance is thus an interactive application-specific reli- pertinent operational, process, and device information.
ability parameter. It is not a typical data sheet specifica- The model then, after calculating the essential data tion, such as a parametric AC/DC specification with points, returns this information to the main program to be benchmark standards for measurement. Microchip formatted and displayed both textually and graphically.
has done extensive predictive laboratory studies on Applying the predictive data to the high endurance block Microchip 2- and 3-wire Serial EEPROMs. These stud- of the 24C65, using the 24LC04 which has similar ies led to the concept of using the computer to predict the characteristics, and assuming the following: theoretical wear out of the floating gate and ultimately to project the point in time of a product’s life cycle when the • an expected E/W cycles of 10 times per day first non-volatile memory bit or periphery failure should occur. After many man-years of data collecting, predict-ing and verifying the results, Microchip feels confident inpublishing and offering for the general technical commu-nity this predictive model in the form of IBM® PC-compatible software. Microchip has a patent pending onthis predictive mathematical model.
Using Endurance Predictive Software
Operational specifications:
endurance of a particular EEPROM device within a setof application parameters. Trade-off analysis can be painfully time-consuming and only marginally accurate without specific knowledge of the behavior of the deviceunder different conditions of use.
The Microchip Total Endurance Software allows the designer to trade off voltage, temperature, write cycles, number of bytes written, number of writes per day, PPMand FIT rates, and years of use in order to optimize the system and accurately predict product lifetime and reli- The following is an example using the Endurance Soft- The 4K HE block with 1 M E/W cycles typical, in this ware to aid in the design of an electronic phone book/ application, should yield the following results: The auto-dialer may have new numbers added orchanged several times per day; but how can the manufacturer specify the life of the unit, and at whatrate of update of the phone numbers? First, thedesigner must make some assumptions. If we as- sume that the average user will change or add 50phone numbers per day, and the manufacturer iswilling to live with a 0.1% failure rate (1,000 PPM) after 24LC04, 25C, 5V, 11bytes, 10cycles/day, RANDOM, BYTE 10 years of use, then we have almost enough informa-tion to verify whether we are in the ball park given the physics of the EEPROM device which will store thenumbers. We also need to know the operating voltage and temperature of the application; we will say that a3.3V lithium button battery is powering the unit and the temperature range is limited to that for which the LCDdisplay will function: 0°C to 70°C. End-of-life voltage for the battery is approximately 2.0V; assuming thatthe ASIC or microcontroller in the application willoperate down to 2.5V, the EEPROM also has a 2.5V requirement. The designer would like to be able tostore 100 phone numbers of 16 bytes each, which results in a 1.6K byte requirement for the SerialEEPROM. Because 1.6K bytes is equal to 12.8K bits,a 16K bit 2-wire Serial EEPROM will more than The results shown are predictive in nature and should suffice. Specifically, Microchip’s 24LC16B will oper- reflect an accurate representation of the expected re- ate down to 2.5V and even includes a write-protect sults. For a more detailed description of endurance, see feature which can be used to block inadvertent writes the related application notes AN536 and AN537 con- tained elsewhere in this volume. All operation param-eters, along with the process technology, effect the effective endurance of a non-volatile device. The volt- age, temperature, cycles per, bytes per cycle, and eventhe number of times written per day (time between write cycles) all have an effect on the oxide breakdown or periphery failure rate of a particular non-volatile process.
Endurance is not a well-defined concept within the semiconductor industry. The number of erase/writecycles which a particular EEPROM can endure is de- pendent not only upon the design of the device but alsoupon the application environment in which it is used.
Therefore, blanket claims such as “1 million erase/writecycles typical” can only be validated based upon thespecific parameters of each application. Yet until now,there has been no tool available for predicting the Using Endurance Predictive Software
Once these values are entered into the Total Endurance for the application life of the product given a 1000 PPM Now we have some more options: (1) specify the Both of the lists above were copied directly from the product life at 5 years or (2) trade-off other parameters Total Endurance program output to the Microsoft® Win- of the application such as voltage or temperature, or (3) dows clipboard and pasted into this document (the Total decide which is more important – a 10-year product Endurance program has a handy menu click to make this lifetime, or the ability to change 50 numbers every single day. Maybe this analysis has caused our designer to re- Unfortunately for our designer, the desired 0.1% failure evaluate the 50 cycle-per-day requirement. Will the user rate has almost doubled to 0.18% (1842 PPM). But really change or add that many numbers per day – half fortunately for the designer, the Total Endurance pro- of the unit’s total capacity? Maybe 20 or even 10 is a gram makes trade-off analysis very simple and fast. At more practical figure. Realistically, a user may enter or this point there are at least three options: (1) live with change quite a few numbers the first week or two of the almost 2000 PPM, or (2) look at the endurance plot and application, and after that the unit will be used mostly for check whether there is a reasonable number of E/W cycles which will provide a 1000 PPM failure rate, or (3) Changing the number of erase/write cycles to 20 per day specify a PPM rate to the Total Endurance program and let it crank out the number of cycles it will take.
Below is the endurance plot, again pasted directly from You can see that by reducing the number of cycles fromthe 182,500 which resulted from our first trial to about 100,000, we can achieve a PPM rate of about 1000 (0.1%). But how does 100,000 cycles translate intoapplication life or cycles per day? By switching the Total Endurance program mode to a PPM request mode instead of application life mode, we can query the program for this information. Let’s ask it Using Endurance Predictive Software
The new PPM rate of 625 gives our triumphant designermore than 30% margin on his PPM target of 1000.
This example shows the significant reduction in time for design trade-off analysis and time-to-market which can be achieved with a useful tool like the Microchip Total Wow! Reducing the number of cycles per day not only Endurance Disk. In addition, it demonstrates the in- brought us back to a 10-year life, it gave us some margin crease in robustness of the system design by providing on that, too. Keeping all the other parameters the same known quantities and readily accessible handles to and forcing a 10-year lifetime gives us the following final modify those quantities in the trade-off analysis. This tool can literally reduce weeks of effort into a few minutesof point and click.
Richard J. FisherMemory Products Division WORLDWIDE SALES & SERVICE
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