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out-of-ram moving from 873 to 876

 
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Dave S
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out-of-ram moving from 873 to 876
PostPosted: Tue Dec 31, 2002 4:02 pm     Reply with quote

I'm trying to move code from a 16F873 to a 16F876. It compiles fine for the 873, but I'm running out of room. When I try to compile for the 876, I get a not-enough-RAM-for-variables error. The largest declare is a 16-byte buffer, certainly nothing near the 96-byte page boundary. Same problem occurs when trying to compile it for the 877. Is there some issue with how the RAM is chopped up on the 876 and 877 parts? I haven't tried the *=16 on the device yet, I'd rather not use the extra ROM if I don't have to. I'd just like to understand what the problem is.
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Original Post ID: 10361
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Re: out-of-ram moving from 873 to 876
PostPosted: Tue Dec 31, 2002 4:30 pm     Reply with quote

:=I'm trying to move code from a 16F873 to a 16F876. It compiles fine for the 873, but I'm running out of room. When I try to compile for the 876, I get a not-enough-RAM-for-variables error. The largest declare is a 16-byte buffer, certainly nothing near the 96-byte page boundary. Same problem occurs when trying to compile it for the 877. Is there some issue with how the RAM is chopped up on the 876 and 877 parts? I haven't tried the *=16 on the device yet, I'd rather not use the extra ROM if I don't have to. I'd just like to understand what the problem is.
-----------------------------------------------------
The 16F876 and 16F877 have 16 bytes of common ram at the
end of each bank. The compiler uses at least 8 bytes of
this ram for internal variables. So if you only have the
first two banks enabled (ie., you're not using *=16), then
you really only have 80 + 80 + 8 = 168 ram locations available
(approximately).

But with the 16F873, it doesn't have the common ram area.
The compiler will still use some of the ram for internal
variables, but it won't affect the other bank. So the
available ram would be 96 + 96 - 8 = 184 bytes (approximately).

So you will lose 16 bytes (184 - 168 = 16) when you move to
the 16F876 or 16F877. You either need to cut your ram usage
or enable the 16-bit ram pointers.
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This message was ported from CCS's old forum
Original Post ID: 10365
Dave S
Guest







Re: out-of-ram moving from 873 to 876
PostPosted: Tue Dec 31, 2002 4:47 pm     Reply with quote

:=-----------------------------------------------------
:=The 16F876 and 16F877 have 16 bytes of common ram at the
:=end of each bank. The compiler uses at least 8 bytes of
:=this ram for internal variables. So if you only have the
:=first two banks enabled (ie., you're not using *=16), then
:=you really only have 80 + 80 + 8 = 168 ram locations available
:=(approximately).
:=
:=But with the 16F873, it doesn't have the common ram area.
:=The compiler will still use some of the ram for internal
:=variables, but it won't affect the other bank. So the
:=available ram would be 96 + 96 - 8 = 184 bytes (approximately).
:=
:=So you will lose 16 bytes (184 - 168 = 16) when you move to
:=the 16F876 or 16F877. You either need to cut your ram usage
:=or enable the 16-bit ram pointers.

Thanks for the speedy reply. I enabled the 16-bit pointers and it seems to work fine. I appreciate the explanation.
Have a Happy New Year!
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Original Post ID: 10369
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