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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
16 kbit 2k x 8 28HC16 At 300 mW 35ns
64 kbit 8k x 8 28C64 At, Se, Xi 120 mW 200ns
64 kbit 8k x 8 28HC64 At 280 mW 45ns
256 kbit 32k x 8 28C2S6 At, Se, Xi 150 mW 200ns
256 kbit 32k x 8 28HC256 At 350 mW 70ns
1 Mbit 128k x 8 28C010 At, Se, Xi 300 mW 200ns
1 Mbit 64k x 16 28C1024 At 400 mW 150ns
Flash EEPROMs, CMOS: (VDD = 5V, f = fmax)
256 kbit 32k x 8 28F256 In, At, Am 150mW 150ns
512 kbit 64k x 8 28F512 In, Se, Am 150mW 150ns
1 Mbit 128k x 8 28F010 In, Se, Am 150mW 150ns
RAMs with subordinate EEPROMs: (VDD =5V)
512 bit 128k x 4 X22C13 Xi   120ns
4 kbit 512k x 8 X2004 Xi 400 mW 200ns
16 kbit 2k x 8 X20C16 Xi   120ns
64 kbit 8k x 8 PNC11C68 Pl 250 mW 35ns

 

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.

 

 

Date: 2015-01-12; view: 1036

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