Flash Microcontrollers Solve Problems
ROM equivalent. In today's technology, the differential in physical size is reduced to about 1.3 times. Thus, the
physical size penalty for using FLASH as opposed to ROM has been essentially eliminated.
In addition to the narrowing of size differential, integrated flash modules are taking a smaller percentage of the
silicon area. FLASH modules are becoming smaller due to reductions in line widths and other processing
improvements. However, improved FLASH EEPROM design has reduced the number of transistors per cell
allowing for further reduction in module size. More hardware peripherals are being incorporated onto
microcontrollers. However, the physical size of the FLASH modules has, in general, not increased as fast as the
other hardware. Thus, the cost of the FLASH on a chip as a percentage of the manufacturing cost is dropping.
Figure 1 and figure 2 show an MC68HC908GP20 using "1T" technology and MC68HC908GP32 using "SST"
technology, respectively. The newer "SST" design not only uses smaller device geometry but implements a more
compact EEPROM cell design than "1T" flash. The 20 Kbyte "1T" flash of the MC68HC908GP20 takes up 36% of
the die area while the "SST" 32 Kbyte flash of the MC68HC908GP32 only takes up 18% of the die area.
FIG. 1 68HC908GP20 36% of die is 1T FLASH