Program a microcontroller over its physical memory limit!!!

Hi all,
my idea is to try to program a uC with a program bigger than it's FLASH memory. Let's consider some uC family where some devices have the same architecture, the same peripherals and the only difference is the RAM and FLASH size. What I am asking to myself is: "Is it possible that same uC of the same family have physically the same RAM and ROM, but it's allowed to the programmer to use only the part declared and sold? let's say similar to some graphic cards in which you can unlock pipelines and exploit the whole hardware. 2 STM8 different only for program memory size are really built with a different memory size? or is it the same but they allow to compiler and programmer to use the declared one?
In case it should be possible to compile the code for the biggest micro and in some way to force flashing the smaller one.

Thanks in advance for your comment, information and precious time

Bye all

Re: Program a microcontroller over its physical memory limit

it means , say , a uc has 64k flash inside . but , they sell it as 8k or 16k etc .

if that is true then all uc of same should have same die area
irrespective of program mem (or ram) size.

bigger here may mean reduced yield , which i dont think , they will do that .

methodology of graphic card may not be applicable here.

earlier what they used is :
make a uc , say , 4k prog mem .
if in QC , the mem part fails , then they disable the mem and sell it
as romless version.


the same here , mem will be enabled for the addresses it what was meant for.

but the current practice in fab can be known from other posters of forum who knew the current situation.

srizbf
12thmay2010

Re: Program a microcontroller over its physical memory limit

I agree with you, moreover memory occupies a big percentage of the area in a uC. So it's not convenient for manifacturers to waste costly silicium.
Anyway some programmers told me they were able to wrote some dummy code
and after the generation of the hex file to manipulate it in order to put this code over the declared limit of the flash memory. I don't have the detail if they put the hex code immediately after the last "nominal" flash memory address or more far.
I'll ask to have more info and I will inform you about. Only I know they were working on an STM8. Moreover I'll try to do the same experiment. Now the only supposition I can do is that there is a spare memory over the memory bound, in case same location are damaged and substituted with spares now, similar to flash memory and other staffs.

Please can you give me some hint about in which section of the forum I can ask info about production process and uC hardware details?

Re: Program a microcontroller over its physical memory limit

I have worked with a very large IC manufacturer who made DRAMs. They quite cleverly built one-time fusible links into the chip so that not only could some regions be permanently disabled but the internal mapping could be changed as well. For example, they would make 1Mb memories which were all OK and sell 512Kb memories where either the bottom or top half of the 1Mb was faulty. The top half was mapped to the lower addresses if there was a fault in the bottom half. So all devices were fully in specification even though the silicon had a defect.

I should point out that there was no cheating involved, this was just a way of utilizing devices that would otherwise have to be scrapped. There was never any possibility of a defective memory being sold or the bad half being reactivated. Most times the defect was a single address that failed only at minimum or maximum temperature and minimum or maximum supply voltage or a combination of these or failed to meet R/W timing requirements. All the devices were 100% tested under all conditions. A typical failure would be one address failing access time at 70C and 5% below specified minimum supply voltage.

Brian.

Hi all,
I had confirmation from some collegue who did it.
There are some microcontroller families from ST, Renesas, and maybe other companies, which are made with the same ram and flash capacity. For example ST produces 2 different microcontroller of the same family with 2 different declared memory size (let's say 4k and 8k) But for real they are both 8k. The second half of the memory in the former is fully accessible but it's not tested or guaranteed.

Nino

Re: Program a microcontroller over its physical memory limit

thank you for your update.

srizbf
20thmay2010

Re: Program a microcontroller over its physical memory limit

Ninochip said:

................
There are some microcontroller families from ST, Renesas, and maybe other companies, which are made with the same ram and flash capacity.

Click to expand...

And many more manufacturers. It is common practice to sell the same die with different memory sizes because it is so incredibly expensive to make a complete new chip.

Ninochip said:

For example ST produces 2 different microcontroller of the same family with 2 different declared memory size (let's say 4k and 8k) But for real they are both 8k. The second half of the memory in the former is fully accessible but it's not tested or guaranteed.
Nino

Click to expand...

Now if you can actually access the upper part of the memory, that is not very smart of the manufacturer. There are ways to disable part of the memory rather easily IF there were plans for that during the design.
It is wasting silicon to sell a chip that has 8k for the price of a 4k chip but it is wasting much more money to do 2 different chips early on. Once the device is mature and no more bugs should be found, the manufacturer can still reduce the die size and make a real 4k chip.
That is how it is usually done when several devices have the same feature set and the same pinout, just different memory sizes.

hth, Bob

Re: Program a microcontroller over its physical memory limit

from manufacturer aspect in focus to reduce the cost all family has the same
wafer mask. Members with 4k flash in examp. have 4k flash tested the other part of
mem. is not tested, and 4k member name. Thats a common proceduce in examp in Zailog. The other are not far from that ..

regards Keen