Re: general questions regarding ARMs


Tauno Voipio wrote:
> The JTAG interface is a backdoor into the processor chip at
> the boundary between the processor core and on-chip peripherals.
> On an AT91xxxxx chip this is the boundary between the ARM part
> and Atmel part of the chip.
>
> You can read and write the processor registers and make
> the processor run single instructions or start the processor
> running. The JTAG loaders use this to write a
> Flash writing program and the desired Flash contents into
> the system RAM and then use the writing program to copy
> the contents into the Flash chip.

Interesting. Is this how a boot loader would copy code to an ARM as
well?

> It's a bit more complicated with ARMs. The processor starts
> by fetching the first instruction at address 0 which should
> for this purpose be in permanent memory (e.g. Flash). On the
> other hand, the processor exception vectors are in the lowest
> 8 fullwords (32 bytes) of memory, and it's desirable to have
> them in writable memory.
>
> The dilemma is solved by the EBI (External Bus Interface) which
> locates the first chip select (CS0-) at address zero after a
> hardware reset. The chip select registers can then be programmed
> by the Flash code, and the new values taken into use with the
> Cancel Remap EBI command. The change is a bit tricky, as it's
> not desirable to lose the code memory during the switch.

Sounds quite complicated - I think I like the AVR initialization
process better :)

> The memory is too small even for an ucLinux version. Of the
> Atmel chips, full Linux needs a memory mapping unit, and
> only AT91RM9200 has one.

Are there any operating systems designed for some of the less powerful
ARMs? Or am I stuck writing everything on my own? I should mention
almost everything that this chip will be doing will be real time.

> The PDC makes it possible to run autonomous serial block
> I/O on the Atmel chips. It's not a general DMA unit.
>
> On the ARM7TDMI, much of the DMA-like functionality can
> be implemented using the FIQ Fast interrupt.

Excuse my ignorance, but what is the difference between this and a
normal DMA unit?

> Welcome - an ARM is not an impossible beast to tame.
> Been there - done that.
>
> HTH
>
> --
>
> Tauno Voipio
> tauno voipio (at) iki fi

Thanks,

-Michael

.

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