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I2C Edge Conditions:
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====================
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I2C devices may be left in a write state if a read was occuring
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and the CPU was reset. This may result in EEPROM data corruption.
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The edge condition is as follows:
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1) A read operation begins.
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2) I2C controller issues a start command.
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3) The I2C writes the device address.
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4) The CPU is reset at this point.
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Once the CPU reinitializes and the read is tried again:
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1) The I2C controller issues a start command.
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2) The I2C controller writes the device address.
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3) The I2C controller writes the offset.
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The EEPROM sees:
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1) START
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2) device address
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3) START "this start is ignored by most EEPROMs"
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4) device address "EEPROM interprets this as offset"
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5) Offset in device, "EEPROM interprets this as data to write"
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The device will interpret this sequence as a WRITE command and
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write rubbish into itself, i.e. the "offset" will be interpreted
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as data to be written in location "device address".
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Notes
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-----
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!!!THIS IS AN UNDOCUMENTED I2C BUS BUG, NOT A AMCC 4xx BUG!!!
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This reset edge condition could possibly be present in every I2C
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controller and device available. For boards where a I2C bus reset
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function can be implemented a i2c_init_board() function should be
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provided and enabled by #define'ing CONFIG_SYS_I2C_INIT_BOARD in your
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board's config file. Note that this is NOT necessary when using the
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bit-banging I2C driver (common/soft_i2c.c) as this already includes
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the I2C bus reset sequence.
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Many thanks to Bill Hunter for finding this serious BUG.
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email to: <williamhunter@attbi.com>
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Erik Theisen <etheisen@mindspring.com>
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Tue, 5 Mar 2002 23:02:19 -0500 (Wed 05:02 MET)
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