32Kbyte Pseudo-Rom

EE JULY/AUGUST 1987 This versatile, exchangeable, memory module should appeal to programmers developing software for computers other than the one being used for writing, testing and debugging the program. The battery back- up function of the module ensures that data is retained, and so makes it possible to use "portable", software that is ROM-based and yet can be altered readily without having to program and erase an EPROM a number of times. The memory module is based on the use of a Type 43256 32 Kbyte static CMOS RAM from NEC-see Fig. 1. Other 32 K types, such as the 62256, should also work here. A bat- tery (2 button cells, or a 2.4 V NiCd cell when DI is bypassed with a resistor to enable charg- ing) enables the chip to retain its contents when the computer is off. When the +5 V supply from the computer is on, Ti drives pin 1 of N5 high, so that this gate can enable the RAM via the CE input. The supply set up around Ts T2 then feeds all the chips on the board with about 4.8 ...