Handy Board FAQ Answers: “moremem”
Is it possible to expand the Handy Board\\\'s memory to 64K?
This is tricky, because the HB\\\'s peripherals (motor outputs, sensorinputs, and expansion interface) are mapped across a full 16K of the lower 32K of memory (see the H\\\'s memorymap).
But, the guys working on simpleRealTimeJava for the 68HC11 have come up with a clever design that alternately maps an additional 32K or the peripherals into the lower half of RAM.
The instructions for modifying your Handy Board to have the extra 32K of RAM are available at http://www.rtjcom.com/6811/hb64.html.
Please note that Interactive C will not support the extra 32K. You\\\'ll have to program your HB in the SimpleRTJ or assembly language to take advantage of the extra RAM installed with this method. Interactive C will continue to work properly on a modified board, however, using the stock 32K memory.
Below is older information about “why it\\\'s hard” to dothe full-blown modification. I guess it\\\'s not so hard if you think about it right :-). The SimpleRTJ solution is agreat hack for improving the existing Handy Board design.
Actually, this is hard to do. Here\\\'s why:
There is a compromise in the design that traded off a wasteful use of the lower 32K of memory for fewer chips used increasing 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, theaddressing circuit that drives the \\\'HC138 chip (which then drivesthese 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 bemapped to the lower 32K bank not used by this remapped \\\'HC138 circuit.
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.
Also, 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.
(Last updated 2002-07-03)