PROGRAMMABLE READ-ONLY MEMORY (PROM) BASIC INFORMATION

Fusible link PROM has now largely been superseded by ultraviolet erasable programmable read-only memory (UVEPROM) and electrically erasable programmable read-only memory (EEPROM). While fusible link devices are effectively permanent, UVEPROM and EEPROM have expected data retention times of 10 to 40 years at room temperature; this has implications for system reliability so they may not be suitable for some systems like those that are exposed to very high temperatures or radiation, such as satellites.

UVPROM and EEPROM use floating gate FETs as the programmable elements. These operate like a normal FET except the gate structure contains an extra isolated conducting layer, the floating gate, which forms a capacitor that can be charged by application of a much higher voltage than used for normal operation.

The effect of charging the capacitor is to change the threshold voltage of the FET. In the uncharged state, the floating gate prevents the FET from turning on when the row line is pulled high, and does not pull the column line low. Once the floating gate is charged the FET can be turned on, pulling the column line low. FLASH memory is based on similar physical effects but the logical architecture is different.

The charge will remain on the capacitor until it leaks away over time, taking 10 to 40 years at room temperature; this leakage can be accelerated by exposure to ultraviolet (UV) light or a high voltage. UVEPROMs are designed to be erased by exposure to short-wavelength UV radiation for about 20 minutes.

It should be noted that the device will be erased by leaving it in direct sunlight for a few days, or under bright fluorescent light for a few months to a year. The package has a quartz window (Figure 10.2) to allow the light in, and this should be covered with a lightproof label if the device is likely to be exposed.

UVEPROMs are available without the window in the package, and these devices are referred to as one time programmable (OTP) devices. The silicon die is identical to that used in the windowed part but the cost of the package is lower.

Microcontrollers are often provided in UVEPROM for development work and in OTP for production. EEPROM do not need the window because they have additional circuitry to erase/re-write the bits.

Fusible link memories are permanent and they can not be reprogrammed, although it is sometimes possible to design a program arrangement so that sections of program can be bypassed by blowing more fuses. The reason that the no-operation (NOP) instruction of some older microprocessors is FFH is to allow changes to programmable devices that cannot be erased.

An instruction can be changed to NOP by blowing all the unblown fuses of a byte. Modern microcontrollers often use 00H as the NOP instruction for the same reason, since OTP versions of UVEPROMs allow program code to be deleted by programming all the bits of a byte.

Small-memory devices of up to about 256 bytes could be made in a similar way to the 8-byte example shown in Figure 10.1, however, as memory devices get larger the address decoding overhead becomes an issue. Square arrays of memory cells are more efficient in their use of silicon.

Using 8 square arrays, one for each bit of the byte, reduces the decoding requirement from 4096 row drivers to 512 row drivers and 512 column lines, making the whole device smaller and nearer a square in shape which makes layout of the row and column interconnect easier.