3.4 Memori element with electrical erase Electrically erasable proMs (eeproMs)

The term EEPROM (electrically erasable PROM) refers to a PROM which, unlike the EPROM, can also be erased electrically. With the more recent types, the voltage converter for generating the programming voltage and the timer for determining the programming pulse duration are incorporated in the same chip. In order to program a byte, merely the address and data must be applied. If programming is then initiated by a write command, the EEPROM stores the address and data internally and immediately re-enables the address and data lines. The subsequent process takes place autonomously on the chip. The old byte is first erased and the new byte is then programmed. This process is internally monitored to ensure that the programmed charge is adequate. The process lasts 1 to 10ms, i.e. the same order of magnitude as with EPROMs. Some EEPROMs are capable of storing not just one byte but an entire "page" containing 16 to 64 bytes in one programming process. To do this, the page is read into an internal RAM before issuing the programming command. This gives effective programming times of 30 us per byte.

However, despite these simple, fast erase and write procedures, one should not be tempted to use an EEPROM as a RAM. The number of possible write cycles is in fact limited: no byte must be written more than 10⁴ to 10⁶ times (depending on type). With a programming time of 1 ms, the end of the operating life of a byte or page may be reached in as little as 10 s if continuous program­ming is performed [3.11, 3.12].

For this reason EEPROMs are sometimes combined with RAMs. The contents of these types of memory are only transferred to the EEPROM if the supply voltage fails. During normal operation, the RAM provides a short write cycle which is not associated with reduction in lifetime [11.13].

Some examples of EEPROMs are listed in Fig. 3.26. With many EEPROMs, as with most memories, the power dissipation is reduced when they are not selected by CS = 1. The smallest power dissipation obviously occurs when the supply voltage is disconnected completely. This does not result in the data being lost - as in the case of all ROMs - but the access time after application of the supply voltage is increased due to the transient response of the read amplifiers. For this reason it is inadvisable to switch on the supply voltage only when the memory is accessed.

Capacity	Organization	Type	Manufacturer	Operating power, typical	Access time, max	Pins
Standard EEPROMs, CMOS: (VDD = 5 V, f = fmax)
16 kbit	2k x 8	28C16	At,Se,Xi	100 mW	200ns	24
16 kbit	2k x 8	28HC16	At	300 mW	35ns	24
64 kbit	8k x 8	28C64	At, Se, Xi	120 mW	200ns	28
64 kbit	8k x 8	28HC64	At	280 mW	45ns	28
256 kbit	32k x 8	28C2S6	At, Se, Xi	150 mW	200ns	28
256 kbit	32k x 8	28HC256	At	350 mW	70ns	28
1 Mbit	128k x 8	28C010	At, Se, Xi	300 mW	200ns	32
1 Mbit	64k x 16	28C1024	At	400 mW	150ns	40
Flash EEPROMs, CMOS: (VDD = 5V, f = fmax)
256 kbit	32k x 8	28F256	In, At, Am	150mW	150ns	32
512 kbit	64k x 8	28F512	In, Se, Am	150mW	150ns	32
1 Mbit	128k x 8	28F010	In, Se, Am	150mW	150ns	32
RAMs with subordinate EEPROMs: (VDD =5V)
512 bit	128k x 4	X22C13	Xi		120ns	18
4 kbit	512k x 8	X2004	Xi	400 mW	200ns	28
16 kbit	2k x 8	X20C16	Xi		120ns	24
64 kbit	8k x 8	PNC11C68	Pl	250 mW	35ns	24

Fig. 3.26 - Examples of EEPROMs.

Manufacturer: Am = AMD, At = Atrnel, In = Intel, Pl = Plessy, Se = Seeq, Xi = Xicor

Flash EEPROMs are intermediates between EPROMs and EEPROMs. Like EEPROMs they can be erased electrically, not byte by byte, however, as can the EPROMs, but the entire chip is erased at once. This is the reason for their name. They are erased much more simply than EPROMs: a single erase pulse lasting for a few seconds is required. It is not necessary to take the package from the circuit and put it for 20 min into an eraser unit. Flash EEPROM technology is little more complicated than that of EPROMs. Correspond-tingly large integration densities consistent with low prices can therefore be achieved. In order to arrive at economical solutions, the converter for the programming voltage and the timer for the programming process normally incorporated in EEPROM devices is omitted in flash EEPROMs. Flash EEPROMs are therefore programmable in the same way as EPROMs.

A comparison of the write and read performance of the various ROM types with that of RAMs is shown in Fig.3.27. We can see that the strength of RAMs is their fast write and read processes which can be repeated any number of times. With all the ROM variants, writing is subject to more or less severe limitations, although all ROMs have the advantage of retaining their contents even in the absence of supply voltage. This characteristic can be achieved for RAMs by adding a buffer battery. As we can see from Fig. 3.8, the current drain of many CMOS-RAMs is generally lower than the self-discharge of a battery. Hence, data retention of 10 years can be ensured by using appropriate batteries.

Fig- 3.27 - Comparison of RAMs and ROMs in terms of their write and read performance.