A simple question..

[Wouter de Waal]

[deleted]
>I ask this as the chip I'm using (80C552, from 8051 family) outputs the
>same memory address range for the ROM and RAM, with 3 control lines:
>
>No. 1 --> Read the program memory (IE the EPROM, this signal is !PSEN)
>No. 2 --> Read the data memory    (IE the RAM,    "     "    "  !RD)
>No. 3 --> Write to the data memory(IE the RAM,    "     "    "  !WR)
>                                               
>IE, you can read location 1234 in EPROM with the !PSEN signal active,
>or   "   "   "      "      "   "  RAM    "    "  !RD     "      "
>                   __
>For me to derive a CS input for each memory device would be a major
>pain, so do I need it or can I just always have them selected?
I've done this before, using 64K EPROM and 32K RAM. Enable the EPROM
/CS and tie /PSEN to /OE. Use A15 for the RAM /CS, and /WR to R/W. Use A15 
inverted for IO devices.

>
>Also, what do you need to do turn a RAM into a non-volatile?
>Just disable the !WR signal when voltage drops below 4.8ish volts, so
>the micro going haywire won't write over the ram? (Obviously with a
>battery / diode arrangement to keep power on the ram),
Did this too... (Same board). Used a 4001 or something similar to gate the 
write signal. The 4001 needs Vbatt too, of course...

>
>OR would it just be easier to use a proprietary (eg DALLAS) nvram module,
>or maybe even flash eeprom? Assume data won't change much.(Maybe 2 or 3
>writes max per day).
I can get hold of the circuitry to use flash. Using some skullduggery it's 
possible to execute out of the same flash that you can program (You run out 
of RAM while programming the FLASH). So you have the RAM for temporary data 
storage, and the flash for programs that can be downloaded. I don't think 
this is exactly what you want to do, though. The Dallas sockets are nice, 
since you get some with built in clocks. Pretty expensive locally, 
unfortunately.

Wouter
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