collapseos/doc/avr.txt

# Programming AVR chips

(In this documentation, you are expected to have an AVR binary
ready to send. To assemble an AVR binary from source, see
asm.txt)

To program AVR chips, you need a device that provides the SPI
protocol. The device built in the rc2014/sdcard recipe fits the
bill. Make sure you can override the SPI clock because the sys-
tem clock will be too fast for most AVR chips, which are usually
running at 1MHz. Because the SPI clock needs to be a 4th of
that, a safe frequency for SPI communication would be 250kHz.

The AVR programmer device is really simple: Wire SPI connections
to proper AVR pins as described in the MCU's datasheet. Note
that this device will be the same as the one you'll use for any
modern SPI-based AVR programmer, with RESET replacing SS.

The AVR programming code is at B160.

Before you begin programming the chip, the device must be desel-
ected. Ensure with "0 (spie)".

Then, you initiate programming mode with "asp$", and then issue
your commands.

Each command will verify that it's in sync, that is, that its
3rd exchange echoes the byte that was sent in the 2nd exchange.
If it doesn't, the command aborts with "AVR err".

# Access fuses

You get/set they values with "aspfx@/aspfx!", x being one of "l"
(low fuse), "h" (high fuse), "e" (extended fuse).

# Access flash

Writing to AVR's flash is done in batch mode, page by page. To
this end, the chip has a buffer which is writable byte-by-byte.

Writing to the flash begins with a call to asperase, which
erases the whole chip. It seems possible to erase flash page-by-
page through parallel programming, but the SPI protocol doesn't
expose it, we have to erase the whole chip. Then, you write to
the buffer using aspfb! and then write to a page using aspfp!.
Example to write 0x1234 to the first byte of the first page:

asperase 0x1234 0 aspfb! 0 aspfp!

Please note that aspfb! deals with *words*, not bytes. If, for
example, you want to hook it to A!*, make sure you use AMOVEW
instead of AMOVE. You will need to create a wrapper word around
aspfb! that divides dst addr by 2 because AMOVEW use byte-based
addresses but aspfb! uses word-based ones. You also have to make
sure that A@* points to @ (or another word-based fetcher)
instead of its default value of C@.