Classification
A summary of CMOS embedded memory performances is given in table 10-2. The typical memory bank capacity gives an advantage to the ROM, EPROM, EEPROM and Flash memories, which is directly correlated with the cell area. The reading and writing performances vary very significantly, as well as the retention of data. Dynamic RAM (DRAM) are slow but compact. Static RAM (SRAM) are fast but large. Reading the information from a passive capacitor, such as in DRAM, is much slower that reading the information from the active inverter-based memory such as in SRAM. In contrast, a single trench of stacked capacitor requires much less silicon surface than a 2-inverter memory structure, at the cost of 8 supplementary process steps. The FLASH memories combine a small area, an acceptable reading cycle and interesting non-volatile capabilities, at the price of a slow writing process (1µs). Promising performances are achieved by ferroelectric RAM (FRAM) which are the most advanced of non-volatile challengers. FRAM have endurance writing/erasing cycles comparable to the best memories, with fast reading and writing cycles, and require only two additional process masks.
| Memory type |
Typical Capacity |
Cell area |
Reading |
Writing |
Cycles |
Retention |
Process complexity |
High voltage |
| ROM |
32Mb |
Very small |
Medium |
Impossible |
0 |
No limit |
0 |
no |
| EPROM |
16Mb |
Very small |
Slow |
Extremely slow |
1-10 |
>30 YEARS |
3 |
yes |
| E2PROM |
1Mb |
Large |
Slow |
Very slow |
1E5-1E7 |
>10 YEARS |
4 |
no |
| FLASH |
16Mb |
Very small |
Medium |
Very slow |
1E4-1E5 |
>10 YEARS |
4 |
yes |
| FRAM |
4Mb |
Small |
Fast |
Fast |
1E12-1E15 |
>10 YEARS |
2 |
no |
| eDRAM |
32Mb |
Small |
Slow |
Fast |
>1E15 |
Volatile, needs to refresh |
8 |
no |
| SRAM |
4Mb |
Large |
Very fast |
Very fast |
>1E15 |
Volatile |
0 |
no |
|
| Table 10-2: A classification of embedded memories according to their performances |
