THE MAGAZINE FOR COMPUTER APPLICATIONS
Circuit Cellar Online offers articles illustrating creative solutions
and unique applications through complete projects, practical
tutorials, and useful design techniques.

IS YOU IS, OR IS YOU ISN'T?

by Tom Cantrell

Start ý Earth ToTom ý Name Gameý The Good Old Days ý Flash Forward ý Sources and PDF

The Good Old Days
Up until the last few years, it was pretty easy to pick flash memory and EEPROM chips out in a lineup. Flash memory chips had pages or sectors, parallel interfaces, and were high density and fast. EEPROMs didnýt have pages or sectors, had serial interfaces, and were low density and slow.

But more recently, the situation has become murkier. Indeed, while surfing the ýNet to check out the latest, I was more than a bit surprised by the proliferation of parts permeating every possible combination of speed, organization, endurance, package, interface, and so on. For instance, a serial interface no longer implies a small, slow EEPROM with a plethora of high-density, high-speed serial flash memory chips hitting the market.

The confusion has just gotten worse with the migration of the technologies from standalone memories onto microcontrollers and other integrated chips. Some have flash memory, some have EEPROM, and some have both. But again, conventional wisdom must be held at bay. For instance, Microchip offers PICs with on-chip flash memory featuring a million cycles write endurance, as much or more than that offered by some supposedly sturdier EEPROMs.

So, whatýs the final straw? Punch "flash EEPROM" (with the quotes) into your search engine. Youýll probably end up on Silicon Storage Technologyýs (SST) home page or the home page of one of their many split-gate process licensees, such as Sanyo (see Figure 3).

Figure 3ýItýs two, two, two chips in one with SSTýs split-gate "flash EEPROM" process.

However, you donýt necessarily need a fancy process to turn flash memory into an EEPROM. EEPROM emulation is all the rage amongst flash memory suppliers, with the addition of buffers, control logic, pseudo-simultaneous read/write capability, software drivers, and all manner of clever hacks toward the end of making one transistor act like two.

Donýt even get me started on the other nonvolatile schemesýhybrid devices with multiple die, SRAMs with built-in batteries, shadow RAMs, ferroelectrics, and on the list goes.

PREVIOUS • NEXT

Circuit Cellar provides up-to-date information for engineers. Visit www.circuitcellar.com for more information and additional articles.
For subscription information, call (860) 875-2199, subscribe@circuitcellar.com or subscribe online. ýCircuit Cellar, the Magazine for Computer Applications. Posted with permission.