Using Flash to store data

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

Hi

I tried to search for this in the forums but didn't succeed.

How can I use PIC Flash to store data? (PIC18F2420). Using #ORG? Using #ROM?

Thanks!
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PCM programmer · Joined: 06 Sep 2003 Posts: 21708

What type of data do you want to store ? How much ?

You can use 'const' to put an array into Flash memory. The limit is 256
elements for the 16F PICs, but the 18F have a much higher limit. I'm not
sure what it is, but probably thousands of elements.

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

Hi PCM.

I'm trying to store some large packages of data (400 bytes per package).
PIC will receive this data through serial port (USART), so I have to write to PIC's flash while the code is running. I can't use contants for this reason.

And besides writting the Flash, I need to read and clear, when desired. Just like an extension to PIC's RAM memory.

If i'm not mistaken, the Flash memory can only be erased in large blocks, not byte by byte, is this correct?

Thanks
_________________
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.

Ttelmah · Guest

You really need to think in terms of another solution. I'd suggest you add an external FRAM chip instead.

There are a series of problems:
1) The 'write' life of the internal flash memory, is limited.

2) When writing to it's own Flash, the processor, _has_ to stop executing code. Unless the serial comms isstopped for the entire duration of the write, data will be lost. This is unavoidable.

3) Writing to flash, is _slow_. With an SPI based external FRAM, you could store the data in under a mSec. Depending on the block size of your chip, writing the same data to flash, would take perhaps 50mSec. While writing to the FRAM, serial interrupts, and other code could still run.

Flash, is the 'wrong' solution for what you describe.

Best Wishes

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

So I guess I'll have to take that data in much small sizes, and store in RAM.

Thanks for all help.
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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.

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands