Micro with USB ISP

On 2011-03-25, Stef <stef33d@yahooI-N-V-A-L-I-D.com.invalid> wrote:
> In comp.arch.embedded,
> Simon Clubley <clubley@remove_me.eisner.decus.org-Earth.UFP> wrote:
>> On 2011-03-25, Stef <stef33d@yahooI-N-V-A-L-I-D.com.invalid> wrote:
>>>
>>> Atmel SAM7S can be programmed through USB. Needs a special programming
>>> application that can be downloaded from Atmel.
>>
>> Assuming you are talking about SAM-BA, this requires jumpers setting on
>> the board to force the SAM-BA code to be copied into the first couple
>> of flash sectors.
>>
>> According to a comment in a Atmel hosted tutorial, there is a limit on
>> the number of times the SAM-BA code can be copied (IIRC, 100 times was
>> mentioned).
>
> Yes, once the device is programmed, you need te restore the bootloader
> before you can program it again. For this you need to set the TST pin
> on power up. We do this via an extra pin on our programming connector
> on one product, via a jumper and a standard USB connector on another
> product.
>
> The datasheet claims 10000 write cycles. With the need to first write
> the boot loader, that would imply 5000 programming cycles. More than
> enough for most applications. I have never run into a programming
> problem, not even on a test board.
>

I have now looked further into this.

Since I was unsure about the accuracy of what was said in the Atmel
tutorial, let me start by apologising to the Atmel tutorial author as he
is quite correct; there is indeed a upper limit of 100 times for which
you can be sure that the SAM-BA re-flashing process will work.

In my defence however, I should point out that the required information
to understand the issue is buried in the middle of the errata section
of the AT91SAM7S datasheet (at least on the version I am using, which is
dated Aug 2010).

The issue is not the number of times you can write to a flash sector.
As you say, and as I already knew, there is a 10000 write cycle endurance.

The issue is that after each restoration of the SAM-BA code to flash, the
flash sectors containing the SAM-BA code are locked and there is a endurance
limit of 100 cycles for setting the lock bits.

From the AT91SAM7S datasheet (doc6175):

|40.6.4   Non Volatile Memory Bits (NVM Bits)
|
|40.6.4.1   NVM Bits: Write/Erase Cycles Number
|
|The maximum number of write/erase cycles for Non Volatile Memory bits is 100.
|This includes Lock Bits (LOCKx), General Purpose NVM bits (GPNVMx) and the
|Security Bit.
|
|This maximum number of write/erase cycles is not applicable to 256 KB Flash
|memory, it remains at 10K for the Flash memory.
|
|Problem Fix/Workaround
|None.

Depending on how you read the datasheet, clearing the lock bit and then
it been set again when SAM-BA is reloaded, may be counted as 2 operations
which means that the unlock sectors -> burn own code -> reload SAM-BA
sequence may only be guaranteed for 50 cycles.

Simon.

-- 
Simon Clubley, clubley@remove_me.eisner.decus.org-Earth.UFP
Microsoft: Bringing you 1980s technology to a 21st century world