GTA04 Block V4

Werner Almesberger werner at
Mon Aug 11 22:53:40 CEST 2008

Ian Stirling wrote:
> Interestingly, some of these devices also offer USB. Exactly why this is  
> interesting, I'm unsure :)

USB would allow us to use such an MPU to implement most of the key
functionality of the debug board, i.e., JTAG and the serial console.

Granted, it would be a bit of work, but I'd be rather happy if we
could get rid of the debug board and its fragile attachment.

Unfortunately, it's still uncommon for these MPUs to have a mode for
loading their firmware over USB. The only one I found is the AR91SAM7,
but that one is a bit inflexible in other regards.

Alternatively, a "foolproof" protection of a boot area in Flash that
could in turn set up USB would be suitable. But I haven't found that

My wish list would look like this:

- ARM core
- single supply voltage (except I/O)
- I/O 3.3V-compatible independent of supply voltage
- wide supply voltage range, at least 3.0V-5.5V, better 2.7V-5.5V
- built-in Flash and RAM
- single crystal
- ultra-low current mode with RAM retention
- USB firmware upload, i.e., one of:
  - Flash erase/write protection of the boot area that cannot be
    turned off by firmware after the boot completes (ideally, this
    would be something like page-based "lock down" bits)
  - ROM-based USB loader (preferred)
- no "trapdoor" fuses (e.g., permanent locking of Flash other than
  the boot area, permanent setting of clocks, etc.)
- Full-Speed USB device
- SPI, I2C, UART - the more the merrier
- available in small BGA packages
- available in a number of configurations for chip size, Flash and
  RAM size, and the number if I/O pins
- dirt-cheap
- in mass production
- kept in stock by distributors like Arrow, Digi-Key, Farnell,
  Mouser, etc.

- Werner

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