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113 lines
4.3 KiB
Plaintext
113 lines
4.3 KiB
Plaintext
# Programming AVR chips
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(In this documentation, you are expected to have an AVR binary
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ready to send. To assemble an AVR binary from source, see
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asm.txt)
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To program AVR chips, you need a device that provides the SPI
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protocol. The device built in the rc2014/sdcard recipe fits the
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bill. Make sure you can override the SPI clock because the sys-
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tem clock will be too fast for most AVR chips, which are usually
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running at 1MHz. Because the SPI clock needs to be a 4th of
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that, a safe frequency for SPI communication would be 250kHz.
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# The programmer device
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The AVR programmer device is really simple: Wire SPI connections
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to proper AVR pins as described in the MCU's datasheet. Note
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that this device will be the same as the one you'll use for any
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modern SPI-based AVR programmer, with RESET replacing SS.
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This device should have an on/off switch that controls the
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chip's power for a very simple reason: Because we can't control
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what's on the chip, it could mess up your whole SPI bus when
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RESET is not held low. This means that as long as it's connected
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and powered, it is likely to mess up your other devices, such as
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the SD card.
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You could put the AVR chip behind a buffer to avoid this, but
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an on/off switch also does the trick and satisfies the low-tech
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lover in you.
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# Programming software
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The AVR programming code is at B160.
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Before you begin programming the chip, the device must be desel-
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ected. Ensure with "0 (spie)".
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Then, you initiate programming mode with "asp$", and then issue
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your commands.
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Each command will verify that it's in sync, that is, that its
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3rd exchange echoes the byte that was sent in the 2nd exchange.
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If it doesn't, the command aborts with "AVR err".
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# Ensuring reliability
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The reliability of your communication depends a lot on the
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soundness of your SPI relay design. If it's good, you will sel-
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dom see those "AVR err".
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However, there are worse things than "AVR err": wrong data. Sync
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checks ensure communication reliability at every command, but
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in the case of commands getting data, you might be out-of-sync
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when you receive your result without knowing it! To ensure that
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you're still in sync, you need to issue a command, which might
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spit "AVR err". If it does, your previous result is unreliable.
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Here's an example word that reliably prints the high fuse value
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from SPI devid 1:
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: get 1 asp$ asprdy aspfh@ asprdy .x 0 (spie) ;
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Another very important matter is clock speed. As mentioned
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above, the safe clock speed is 250kHz. If you use the SPI design
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in rc2014/sdcard recipe, this means that your input clock speed
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can theoretically be 500kHz because the '161 divides it by 2.
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In practice, however, you can't really do that because depending
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on the timing of your SPI write, the first "bump" of the SPI
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clock might end up being nearly 500kHz, which will result in oc-
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casional communication errors.
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The simplest and safest way to avoid this is to reduce your
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raw input clock by 2, which will reduce your effective communi-
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cation speed by 2. There certainly are options allowing you to
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keep optimal speed, but they're significantly more complex than
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accepting slower speed.
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# Access fuses
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You get/set they values with "aspfx@/aspfx!", x being one of "l"
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(low fuse), "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
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this end, the 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
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erases the whole chip. It seems possible to erase flash page-by-
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page through parallel programming, but the SPI protocol doesn't
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expose it, we have to erase the whole chip. Then, you write to
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the buffer using aspfb! and then write to a page using aspfp!.
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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
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example, you want to hook it to A!*, make sure you use MOVEW
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instead of MOVE. You will need to create a wrapper word around
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aspfb! that divides dst addr by 2 because MOVEW use byte-based
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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)
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instead of its default value of C@.
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# Access EEPROM
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Accessing EEPROM is simple and is done byte-by-byte with words
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aspe@ and aspe!. Example:
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0x42 0 aspe! 0 aspe@ .x ( prints 42 )
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