[En-Nut-Discussion] Memory Question

[Alastair Jeremy]

Hi Mike,

Short answer: no. (I assume you are talking about on the Ethernut)

Long answer: The ATmega itself always just "looks for" the exact specified
address, using a 16 bit databus (assuming you haven't changed the register
which reduces the number of high bits in the address). If you want to access
the lower 4kb or so then there are two ways of achieving this: one is to use
the method in the ATmega datasheet, and the other is to have a complex
external decoding mechanism which "converts" a particular range of addresses
to actually address the lower 4k of SRAM.

The Ethernut does not have this complex external decoding mechanism. It just
uses the A15 line to select the SRAM or not. So since between 8000-82ff A15
will be high, the SRAM chip will not be selected. If you had some way of
making -CS on the SRAM be asserted during this range, then the addresses
would wrap around like you suggested. Reducing the number of 'upper address
lines' as per the ATmega datasheet would work (and setting -CS asserted all
the time for the SRAM), but then you would lose the ethernet controller.

Alastair Jeremy
DOT AU Pty Limited
http://www.dotaussie.com.au/


-----Original Message-----
From: en-nut-discussion-admin at egnite.de
[mailto:en-nut-discussion-admin at egnite.de]On Behalf Of Mike Baverso
Sent: Friday, 20 December 2002 4:05 AM
To: en-nut-discussion at egnite.de
Subject: [En-Nut-Discussion] Memory Question


Ref: ATMEGA128 datasheet page 31 'Using all Locations of External Sram
Smaller than 64K'

Do addresses 0x8000-0x82FF map directly to 0x0000-0x02FF of the sram???

Regards,

Mike Baverso
Elcon Technologies, Inc.
106 Rochester Road
Pittsburgh, PA 15229

Phone:412 931 8250
Fax:    412 931 8254
Cell:    412 559 4865
mbaverso at elcontech.com
www.elcontech.com

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