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- # Working with AVR microcontrollers
-
- # Assembling AVR binaries
-
- TODO
-
- # Programming AVR chips
-
- 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 system 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.
-
- Because you will not be using your system clock, you'll also need to override
- SPI_DELAY in your xcomp unit: the default value for this is 2 NOP, which only
- works when you use the system clock.
-
- Alternatively, you could run your whole system at 250kHz, but that's going to be
- really slow.
-
- 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.
-
- (TODO: design a SPI relay that supports more than one device. At the time of
- this writing, one has to disconnect the SD card reader before enabling the AVR
- programmer)
-
- The AVR programming code is at B690.
-
- Before you begin programming the chip, the device must be deselected. Ensure
- with "(spid)".
-
- 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@.
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