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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design,comp.sys.raspberry-pi
Subject: Re: RP2040 reset idea
Date: Sat, 21 Sep 2024 19:50:34 -0000 (UTC)
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john larkin <JL@gct.com> wrote:
> On Sat, 21 Sep 2024 19:29:26 +0100, The Natural Philosopher
> <tnp@invalid.invalid> wrote:
> 
>> On 21/09/2024 16:08, john larkin wrote:
>>> On Sat, 21 Sep 2024 09:12:06 +0100, The Natural Philosopher
>>> <tnp@invalid.invalid> wrote:
>>> 
>>>> On 20/09/2024 19:00, john larkin wrote:
>>>>> On 20 Sep 2024 11:30:13 +0100 (BST), Theo
>>>>> <theom+news@chiark.greenend.org.uk> wrote:
>>>>> 
>>>>>> In comp.sys.raspberry-pi The Natural Philosopher <tnp@invalid.invalid> wrote:
>>>>>>> On 19/09/2024 23:09, Lasse Langwadt wrote:
>>>>>>>> On 9/18/24 00:33, john larkin wrote:
>>>>>>> 
>>>>>>>>> It looks like a USB memory stick. You can delete or add files if you
>>>>>>>>> want.
>>>>>>>>> 
>>>>>>>>> It boots CPU 0 (the one we call Alice) from a file with the extension
>>>>>>>>> .UL2
>>>>>>>>> 
>>>>>>>>> Why   .UL2   one wonders.
>>>>>>>>> 
>>>>>>>>> We'll put a bunch of files into the flash. Code for Bob, the 2nd CPU.
>>>>>>>>> An FPGA bitstream file. A prototype calibration table. A README file
>>>>>>>>> to explain everything in plain English.
>>>>>>>> 
>>>>>>>> sure it's not UF2?
>>>>>>>> 
>>>>>>>> https://github.com/microsoft/uf2
>>>>>>>> 
>>>>>>>> 
>>>>>>> Definitely uf2 here.
>>>>>>> 
>>>>>>> And no, you cannot 'delete or add files' to it.
>>>>>>> The action of pretending to download a uf2 file into what appears to be
>>>>>>> an empty drive, erases everything on it and programs the flash.
>>>>>>> 
>>>>>>> There are no visible files to delete.
>>>>>> 
>>>>>> Neat.  So basically you throw some files at it, which causes a series of
>>>>>> block writes.  UF2 picks out specially tagged block writes and uses that to
>>>>>> program the flash.  It doesn't actually care what other stuff is written to
>>>>>> the flash as it ignores all of that, so it doesn't care about all the FAT
>>>>>> stuff or whatever junk your OS decides to put on there.
>>>>>> 
>>>>>> Means you can write any kind of files to it and it'll only pay attention to
>>>>>> the specific tagged blocks.  If the OS is happy to cache the medium (as many
>>>>>> do) you could maybe even reformat it as some other filesystem like NTFS and
>>>>>> it would still handle writing the UF2 file correctly.
>>>>>> 
>>>>>> Theo
>>>>> 
>>>>> My Pi guy says that you can only write one file, and the act of
>>>>> writing that file wipes anything that was there before. So the flash
>>>>> probably doesn't have a file structure, and the USB memory-stick write
>>>>> is, well, a sort of cheap trick.
>>>>> 
>>>>> That's workable, if inelegant. We can pack everything we need into
>>>>> that one big file and users can upgrade box code in the field pretty
>>>>> easily.
>>>>> 
>>>> 
>>>> It gets nastier if you want to preserve config info across reboots.
>>>> It is possible to read and write areas of flash from the code, but its
>>>> no picnic.
>>>> And it gets wiped when new code is uploaded
>>>> 
>>>> 
>>>> It is an area I will have to tackle for one project tho.
>>> 
>>> Yes, writing to flash from the running application is nasty.
>>> 
>>> We have to calibrate each box. We'll store the prototype calibration
>>> table inside the big flash image. At factory test, we'll grab that,
>>> edit it for this particular unit, and save it to a small SPI eeprom
>>> chip. That costs 24 cents and one chip select pin.
>>> 
>>> My guy says that there are a few magic integers at the start of the
>>> UF2 file that identifies it, well, as a UF2 file. That confirms that
>>> the Pico flash doesn't have a file structure, it just stores one giant
>>> chunk of stuff starting at the start.
>>> 
>>> It's Windows who lies about it acting like a USB memory stick that
>>> stores files.
>>> 
>>> We did consider saving the real cal table at some fixed physical
>>> address near the end of the flash , on the theory that nobody will
>>> ever write a bootable image that big. That might work.
>>> 
>> That seems to be the case.
>> 
>> I looked into it enough to see that it would be possible to store NV 
>> data in a high part of the flash.
>> 
>> I think that the runtime provides access to a memory location that 
>> indicates the end of the uploaded flash image, so in theory flash above 
>> that is free to write, with the proviso it has to be done in large 
>> blocks on specific address boundaries.
>> 
>> All this is at least Pi Pico specific anyway.
> 
> We're using the RP2040 chip, so will have a huge flash chip. We will
> sometimes store an FPGA config file that could be too big for the 2
> MByte part on the Pico.
> 
> 
>> 
>> Will keep me busy through the dark winter days...:-)
> 
> Storing anything in high flash still has the problem that you can't
> run flash-cached code while the write is going on, unless you are very
> careful. 
> 
> 

It’s good to have a warm relationship with your linker mapfile. ;)

Cheers 

Phil Hobbs 

-- 
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics