65 lines
2.3 KiB
Plaintext
65 lines
2.3 KiB
Plaintext
drivers/video/README.max7456
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23 March 2019
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Bill Gatliff <bgat@billgatliff.com>
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The code in max7456.[ch] is a preliminary device driver for the MAX7456 analog
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on-screen-display generator. This SPI slave chip is a popular feature in many
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embedded devices due its low cost and power requirements. In particular, you
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see it a lot on drone flight-management units.
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I use the term "preliminary" because at present, only the most rudimentary
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capabilities of the chip are supported:
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* chip reset and startup
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* read and write low-level chip control registers (DEBUG mode only)
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* write CA (Character Address) data to the chip's framebuffer memory
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Some key missing features are, in no particular order:
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* VSYNC and HSYNC synchronization (prevents flicker)
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* ability to update NVM (define custom character sets)
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If you have a factory-fresh chip, then the datasheet shows you what the factory
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character data set looks like. If you've used the chip in other scenarios,
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i.e. with Betaflight or similar, then your chip will almost certainly have had
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the factory character data replaced with something application-specific.
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Either way, you'll probably want to update your character set before long. I
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should probably get that working, unless you want to take a look at it
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yoruself...
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The max7456_register() function starts things rolling. The omnibusf4 target
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device provides an example (there may be others by the time you read this).
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In normal use, the driver creates a set of interfaces under /dev, i.e.:
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/dev/osd0/fb
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/dev/osd0/raw (*)
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/dev/osd0/vsync (*)
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* - not yet implemented
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By writing character data to the "fb" interface, you'll see data appear on the
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display. NOTE that the data you write is NOT, for example, ASCII text: it is
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the addresses of the characters in the chip's onboard character map.
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For example, if entry 42 in your onboard character map is a bitmap that looks
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like "H", then when you write the ASCII "*" (decimal 42, hex 2a), you'll see
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that "H" appear on your screen.
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If you build the code with the DEBUG macro defined, you will see a bunch more interfaces:
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/dev/osd0/VM0
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/dev/osd0/VM1
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/dev/osd/DMM
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...
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...
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These are interfaces to the low-level chip registers, which can be read and/or
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written to help you figure out what's going on inside the chip. They're
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probably more useful for me than you, but there they are in case I'm wrong
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about that.
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b.g.
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