Subject:
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Re: 64K RAM?
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Wed, 15 Apr 1998 17:36:35 GMT
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Original-From:
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Fred G. Martin <fredm@media.SPAMLESSmit.edu>
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Viewed:
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894 times
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This is a common question, so I went ahead and put an answer in the
FAQ. Take a look at:
http://el.www.media.mit.edu/projects/handy-board/faq/index.html#moremem
Here's what it now says:
------------------------------------------------------------------------
IS IT POSSIBLE TO EXPAND THE HANDY BOARD'S MEMORY TO 64K?
Actually, this is hard to do. Here's why:
1.There is a compromise in the design that traded off a wasteful use
of the lower 32K of memory for fewer chips used in creating the motor
output, digital input, and expansion bus circuits. Basically, 16K of
the lower 32K is used to map four input latches and four output
latches to the microprocessor (see more info in the memory map
answer).
What this means is that in order to recapture the lower 32K, the
addressing circuit that drives the 'HC138 chip (which then drives
these latches) would need to be modified. Presently, the 'HC138 is
wired to the 68HC11, using the A15, A14, A13, and A12 signals. To
recapture the address space, many more address signals would need to
be combined to force the HC138 to enable itself only when a narrow
band of addresses were accessed. Then, an additional RAM could be
mapped to the lower 32K bank not used by this remapped 'HC138 circuit.
2.Interactive C
would need to be recompiled. Interactive C was designed with certain
assumptions about the architecture of the target system; one of those
included a 32K RAM. Various modifications to Interactive C (largely,
the redefining of compile-time constants) would be necessary for it to
use a different memory architecture.
This said, it is relatively straightforward to build a
serially-accessible external memory for data storage. For example, see
Jim Fong's SPI-based EEPROM application note.
------------------------------------------------------------------------
In your message you said:
> Hello, I wonder if anyone knows if I can expand the Handyboard's memory
> to the maximum of 64KB simply be wiring in a 128kb SRAM (A16 unused so
> it becomes 64kb) with D0-D7 to the data bus as usual and A0-A14 to the
> existing RAM socket and A15 from the expansion header directly to the
> RAM's A15? Would IC need any modification or anything? Thanks!
>
>
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Message is in Reply To:
 | | 64K RAM?
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| Hello, I wonder if anyone knows if I can expand the Handyboard's memory to the maximum of 64KB simply be wiring in a 128kb SRAM (A16 unused so it becomes 64kb) with D0-D7 to the data bus as usual and A0-A14 to the existing RAM socket and A15 from (...) (26 years ago, 15-Apr-98, to lugnet.robotics.handyboard)
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