How much FLASH memory is free for a program?

Question

I'm programming for a STM32F103C8T6 (aka blue pill), with the be datasheet here.

On page 14 it states:

2.3.2 Embedded Flash memory 64 or 128 Kbytes of embedded Flash is available for storing programs and data.

Now mine has 64 KB, however, does this mean I have 100% available, or is there some part reduced? And if so, how much? Or is there a way to find out when it will not 'fit' (like an error will show up?).

I use an STM32 ST-LINK V2, and afaik I'm not using any special bootloader, development environment is Workbench 4 STM / Eclipse, and using HAL/STMCube.

Currently I'm using only about 19KB but I'm not finished (and running my unit tests also within this code, although I can switch it off, which saves half).

Answer 1

In a typical microcontroller, you can program all of the flash with your code. You can check the TRM to be sure -- if it talks about a boot loader in the first sector or tells you there are parts of the flash you can't erase, then those parts are reserved. But normally you get it all.

Note that that doesn't mean 100% of the flash will be used for code! You'll have interrupt vector addresses at the start of the flash. Constants and initialization values for static variables will also be stored in the flash. Your linker should take care of this for you, probably using a linker script supplied by the manufacturer (ST). The linker should give an error if your program is too big to fit in flash.

Advanced programmers can write their own linker scripts and manually assign code and data to whatever parts of the flash they want. For example, a default linker script from a manufacturer will normally put the interrupt vectors at the start of flash (or RAM, if running out of RAM).

Answer 2

The datasheet is describing a family of parts, not just one. Note that the first page lists the part names as:

STM32F103x8
STM32F103xB

The STM32F103C8T6 is a specific variant with a 48-pin LQFP package and 64 KB of flash, rated for an industrial temperature range. An example of another part covered by this datasheet might be the STM32F103VBH6, which has a 100-pin BGA package and 128 KB of flash.

Refer to page 108 of the datasheet ("Ordering information scheme") for more information on how to decode the part numbers.

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Now, a practical note: many parts marked as "STM32F103C8T6" actually have 128 KB of flash available. (They are, in effect, …CBT6 parts remarked as …C8T6 for inventory reasons.) You shouldn't rely on the availability of this extra flash for a product, but it's a convenient bonus for development. :)

Answer 3

That datasheet is for both STM32F103x8 and STM32F103xB.

You can see in "Table 63. Ordering information scheme" on page 108, that STM32F103x8 has 64 Kbytes flash, and STM32F103xB has 128 Kbytes flash.

The 'x' in the part number identifies the package.

So the STM32F103C8T6 is STM32F103 plus:

• C = 48 pins
• 8 = 64 Kbytes flash
• T = LQFP package
• 6 = Industrial temperature range, –40 to 85 °C

So yours has only 64 Kbytes, but you can use all of that. It is not a part with 128 Kbytes flash.

However, there are stories (eg. at stm32duino.com) of STMicro sometimes shipping parts with 128 Kbytes flash but with STM32F103x8 part number printed on the package, so you might be lucky.

Answer 4

is there a way to find out when it will not 'fit' ?

The IDE or compiler may show you memory (flash or RAM) overflow warning. I have tested with my F3 board.There was once I declared an big array for testing purpose, when I built the source, Eclipse threw me a RAM overflow error.