_OV7670_init registers confusion

[Chris-F5]

This issue is regarding _OV7670_init, an extract from which follows:

...
        _OV7670_REG_COM8,
        _OV7670_COM8_FASTAEC | _OV7670_COM8_AECSTEP | _OV7670_COM8_BANDING,
        _OV7670_REG_GAIN,
        0x00,
        _OV7670_COM2_SSLEEP, # <--- =0x10. This should be a register.
        0x00,
        _OV7670_REG_COM4,
        0x00,
        _OV7670_REG_COM9,
        0x20,  # Max AGC value
        _OV7670_REG_BD50MAX,
...

See how _OV7670_COM2_SSLEEP appears in an even (register) slot. This causes 0x00 to be written to register 0x10 (since _OV7670_COM2_SSLEEP=0x10). 0x10 corresponds to the _OV7670_REG_AECH exposure register with default value 0x40. I'm not sure what the intended behaviour is. I tried removing these two lines and noticed no visual difference in the output image. So its not an urgent problem, just a confusing piece of code that might cause subtle problems.

Also, at some other points in the init sequence it writes to reserved registers like 0x29 and 0x35:

...
        0x29,
        0x07,  # Reserved register?
        _OV7670_REG_CHLF,
        0x0B,
        0x35,
        0x0B,  # Reserved register?
        _OV7670_REG_ADC,
        0x1D,
...

I don't understand why. Perhaps the code should explain in a comment unless I am missing something obvious?

It almost looks like this entire init sequence was copied from some documentation or code elsewhere. Hence the questionmarks. If this is the case then perhaps a comment could link to this.

[jepler]

Yes it almost certainly was copied from elsewhere. https://github.com/adafruit/Adafruit_OV7670/blob/master/src/ov7670.c looks very similar, with these snippets:

        {OV7670_REG_GAIN, 0x00},
        {OV7670_COM2_SSLEEP, 0x00},
        {OV7670_REG_COM4, 0x00},

        {0x29, 0x07}, // Reserved register?
        {OV7670_REG_CHLF, 0x0B},
        {0x35, 0x0B}, // Reserved register?
        {OV7670_REG_ADC, 0x1D},

When I was developing this driver, the advice I received and followed was to copy an existing initialization procedure that was "known to work", rather than trying to create a fresh initialization procedure by reading the datasheet.