[Q]: external RAM for PIC

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Hi,

I read a bit in the manual of the PIC18F8620, and saw it has a 21bit
PC.
But am I right, that a PIC can only access its interal RAM as
data-memory ?
Though it seems strange: 2MB ROM, but only 3840bytes RAM ?

I wonder, because I am investigating if it is possible to port my
company's RTOS, but with 3 to 4k RAM this makes no sense.

TIA
---
42Bastian
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Re: [Q]: external RAM for PIC
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Strange, sure, but that's what this chip was designed to be like.
Actually, if memory serves me well, you should consider yourself
rather lucky you even got as many as 3840 bytes of RAM --- other PICs
have even less.

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It almost certainly doesn't make sense.  

I strongly doubt any RTOS not written explicitly for the PIC can ever
be ported to it.  That platform is just too weird for such an effort
to work out.  In other words: if you want another platform to port
your existing RTOS to, PIC is almost certainly not it.
--
Hans-Bernhard Broeker ( snipped-for-privacy@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.

Re: [Q]: external RAM for PIC
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You'd be wrong, then. I've been involved in developing three RTOSes on
PIC18 hardware, all of which we had ported from more conventional
non-Harvard machines. Much of what we discovered on the PIC helped us to
port to DSP architectures too.

pete
--
snipped-for-privacy@fenelon.com "there's no room for enigmas in built-up areas" HMHB


Re: [Q]: external RAM for PIC
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Three ?! Then I should concentrate on another CPU ;-)

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These are strange too. Right.

So maybe I should start and take it as exercise :-)
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42Bastian
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Re: [Q]: external RAM for PIC
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Becuase the PIC has a Harvard architecture where the code and data
spaces are completely separate as opposed to a Von Neuman architecture
where the code and data share the same space like in most Motorola micros.

IAn


Re: [Q]: external RAM for PIC
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Means the data space can not be expanded ?
---
42Bastian
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Re: [Q]: external RAM for PIC
The data space on some PICs can be expanded with external memory, but you
lose a lot of your ports. Just look for "External Memory" in the datasheets.

Peter

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micros.



Re: [Q]: external RAM for PIC

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I did. And from my understanding you can access data only thru the
table-opcodes.

What I hoped is an expansion to the real data space, i.e. data RAM I
can read directly without the need to use special opcodes.

But I see, it is not (yet ?!) possible to expand the register map.

---
42Bastian
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Re: [Q]: external RAM for PIC
On Thu, 28 Aug 2003 06:28:44 GMT, snipped-for-privacy@yahoo.com (42Bastian


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     Additional data can be read or writtened to EEPROMs, serial
flash, or FRAM chips throught the SPI interface.  If you are looking
to directly address the memory, it probably would be best to consider
a different architecture than PIC.  The PIC was designed for small
embedded control projects and does this job well.  However, it has
never been the best choice for handling large tables or data blocks.
You might take a look at the NEC K0 series if you are looking for an 8
bit that handles external memory well.


Re: external RAM for PIC

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Forget it, PIC is not a good choice.. it's ok for mickey mouse stuff, but
for serious applications, choose something more appropriate..


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Re: external RAM for PIC

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You are making the mistake of equating "small" with "not serious".
There are plenty of small serious applications for which the PIC is
perfectly appropriate, although most of them would have no use for the
OP's RTOS.


-Robert Scott
 Ypsilanti, Michigan
(Reply through newsgroups, not by direct e-mail, as automatic reply address is
fake.)


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