mirror of https://github.com/hsoft/collapseos.git
66 lines
2.6 KiB
Plaintext
66 lines
2.6 KiB
Plaintext
# Working with AVR microcontrollers
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# Assembling AVR binaries
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TODO
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# Programming AVR chips
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To program AVR chips, you need a device that provides the SPI protocol. The
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device built in the rc2014/sdcard recipe fits the bill. Make sure you can
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override the SPI clock because the system clock will be too fast for most AVR
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chips, which are usually running at 1MHz. Because the SPI clock needs to be a
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4th of that, a safe frequency for SPI communication would be 250kHz.
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Because you will not be using your system clock, you'll also need to override
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SPI_DELAY in your xcomp unit: the default value for this is 2 NOP, which only
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works when you use the system clock.
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Alternatively, you could run your whole system at 250kHz, but that's going to be
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really slow.
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The AVR programmer device is really simple: Wire SPI connections to proper AVR
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pins as described in the MCU's datasheet. Note that this device will be the same
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as the one you'll use for any modern SPI-based AVR programmer, with RESET
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replacing SS.
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(TODO: design a SPI relay that supports more than one device. At the time of
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this writing, one has to disconnect the SD card reader before enabling the AVR
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programmer)
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The AVR programming code is at B690.
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Before you begin programming the chip, the device must be deselected. Ensure
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with "(spid)".
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Then, you initiate programming mode with "asp$", and then issue your commands.
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Each command will verify that it's in sync, that is, that its 3rd exchange
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echoes the byte that was sent in the 2nd exchange. If it doesn't, the command
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aborts with "AVR err".
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# Access fuses
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You get/set they values with "aspfx@/aspfx!", x being one of "l" (low fuse),
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"h" (high fuse), "e" (extended fuse).
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# Access flash
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Writing to AVR's flash is done in batch mode, page by page. To this end, the
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chip has a buffer which is writable byte-by-byte.
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Writing to the flash begins with a call to asperase, which erases the whole
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chip. It seems possible to erase flash page-by-page through parallel
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programming, but the SPI protocol doesn't expose it, we have to erase the whole
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chip. Then, you write to the buffer using aspfb! and then write to a page using
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aspfp!. Example to write 0x1234 to the first byte of the first page:
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asperase 0x1234 0 aspfb! 0 aspfp!
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Please note that aspfb! deals with *words*, not bytes. If, for example, you want
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to hook it to A!*, make sure you use AMOVEW instead of AMOVE. You will need to
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create a wrapper word around aspfb! that divides dst addr by 2 because AMOVEW
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use byte-based addresses but aspfb! uses word-based ones. You also have to make
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sure that A@* points to @ (or another word-based fetcher) instead of its default
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value of C@.
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