2 PICs (8-bit), same size and program, different ROM usage?

rhaguiuda · Joined: 05 Sep 2007 Posts: 46 Location: Londrina - Brazil

This is very strange to me.

My compiler version is: 4.057

I'm using a 16F628A in my project, but when the ROM usage hit 99% I changed to 16F870 just for testing ROM usage. The strange thing is ROM drop to 85%. (both PICs have 2kwords).

How is that possible if the only change I made in the code was internal to external oscillator?

I'v also checked the instruction set in both datasheet, and they're same!

So, same instruction set, both 8-bit, same program.. but one is 15% smaller??? How?
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PCM programmer · Joined: 06 Sep 2003 Posts: 21708

What peripheral modules are you using ? They are on different pins
in these PICs. It's possible that on one PIC you are using a hardware
module, but in the other PIC you are using software emulation.
(i.e., CCS can do a hardware or software UART, depending on the pins
that are specified in the #use rs232() statement).

Also, the RAM layout is different. There might be a different allocation
of variables, requiring more bank select instructions.

However, 15% does seem excessive. If you want to know for sure,
you will have to do a side-by-side comparison of the .LST files.

rhaguiuda · Joined: 05 Sep 2007 Posts: 46 Location: Londrina - Brazil

Thanks for replying PCM.

When I put a receive/transmit PIN that is different from PIC's TX/RX pins, the compiler automatically creates a software USART?

Anyway, I don't believe this is the problem, because in 16F628A the pins were correctly assigned to TX/RX pins. If with the 16F870 the compiler was generating a software USART, than the code would be larger, not smaller, isn't?

Well, about taking a look at both .lst files, I have no time to make a side by side comparison, so I took another PIC and move on with my project.

Thanks very much.
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Give a man a fish and you'll feed him for a day. Teach a man to fish, and you'll feed him for a lifetime.

asmallri · Joined: 12 Aug 2004 Posts: 1635 Location: Perth, Australia

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

I prefer "Build a man a fire and he is warm for a day. Set a man on fire and he is warm for the rest of his life." Call it January humor....
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