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51 lines
2.1 KiB
Plaintext
51 lines
2.1 KiB
Plaintext
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# 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$", and then issue your commands.
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At this time, only fuse commands are implemented. You get/set they values with
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"aspfx@/aspfx!", x being one of "l" (low fuse), "h" (high fuse), "e" (extended
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fuse).
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Each command will verify that it's in sync, that is, that its 3rd exchange
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echoes the byte that was sent in the 2nd exchange. If it doesn't, the command
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aborts with "AVR err".
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At the time of this writing, it is recommended that you perform your commands in
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batch, that is, running your commands right after the "asp$" command, ending
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your batch with "(spid)" so that the next batch works. In my tests, interacting
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with the chip "live" in a single "asp$" session sometimes resulted in unreliable
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data that didn't properly detect sync errors. TODO: investigate further.
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