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18F2550 vs 18F27J53 Bootloader size...

 
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spilz



Joined: 30 Jan 2012
Posts: 219

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18F2550 vs 18F27J53 Bootloader size...
PostPosted: Mon Jun 08, 2015 6:04 am     Reply with quote
hello,

I'm using CCS usb bootloader on 18F2550 for a while, and it works well.
the usb bootloader takes : ROM 19% RAM 24-30%

I'm trying to use a 18F27J53 with the same usb bootloader, and it takes :
ROM 5% RAM 39-42%

I don't understand why the RAM increase instead of decrease.

Is it normal ?

thanks for your help

Spilz
RF_Developer



Joined: 07 Feb 2011
Posts: 839

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Re: 18F2550 vs 18F27J53 Bootloader size...
PostPosted: Mon Jun 08, 2015 6:30 am     Reply with quote
spilz wrote:
I'm using CCS usb bootloader on 18F2550 for a while, and it works well.
the usb bootloader takes : ROM 19% RAM 24-30%

I'm trying to use a 18F27J53 with the same usb bootloader, and it takes :
ROM 5% RAM 39-42%

I don't understand why the RAM increase instead of discrease.

Is it normal ?


Without looking at how much RAM is on each PIC, I dont know if it's "normal" but certainly sounds reasonable. To be honest, I'd expect a well written bootloader to take the maximum advantage of the RAM available to it. With more RAM, I'd expect it to buffer more data and write and verify in larger chunks.

You see, it doesn't matter how much RAM the bootloader uses; the application will re-use all of it. Only the ROM used by the bootloader matters, as the code has to live alongside the application code. WiIth RAM, however, either the application uses it, or the bootloader uses it, but they NEVER run at the same time.
spilz



Joined: 30 Jan 2012
Posts: 219

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PostPosted: Mon Jun 08, 2015 7:00 am     Reply with quote
thanks for your reply,

PIC18F2550 :
- Program Memory : 32K (ROM?)
- SRAM : 2048 (RAM?)

PIC18F27J53 :
- Program Memory : 128K (ROM?)
- SRAM : 3.8K* (RAM?)

*Dual access RAM for USB and/or general purpose use

So if I understand well : RAM is bigger in PIC18F27J53

I think you're right about RAM use, as it's never at the same time, it doesn't mater,
But I prefer asking Smile

thanks for your help
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