Re: [Sdcc-user] FW: 8051/EZ-USB FX code segment

[Erik P. <epe...@iv...>]

On Tue, 21 Sep 2004, Muller Jaco <jjm...@su...> wrote:

> I have a very small test program for a EZ-USB FX that switches a LED on.
> If I compile the code using "sdcc --code-loc 0x0000 firmware.c",
> everything works fine. I would like to run the code segment from
> external memory, eg. "sdcc --code-loc 0x4000 firmware.c", but this
> doesn't work. It only works when the code starts at 0x000.
>  
> If I look at the .map and .mem files, everything looks like I expected
> it to be (the code segment starts at 0x4000 instead of 0x0000). Is there
> something else that I need to look at?

You can put the code at a different location, but the processor will still
start execution at 0x0000.

To solve this in the project I worked on, I took advantage of the EZ-USB
FX's I2C EEPROM feature. Besides loading configuration data from the
EEPROM, the FX can also automatically load a program from the EEPROM into
the code RAM. Instead of loading my entire program from the EEPROM, I used
it to load the 3 bytes needed for an ljmp instruction that would jump to
the starting address of my external flash chip.

Alternatively, you can tie the EA pin high and decode your external code
memory chip at location 0x0000. The disadvantage is that the built-in 8K
of RAM is no longer available for code (it becomes xdata only); this may
or may not be a problem depending on your application.

   Erik