89 lines
3 KiB
Text
89 lines
3 KiB
Text
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This document describes the i2c protocol. Or will, when it is finished :-)
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Key to symbols
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==============
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S (1 bit) : Start bit
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P (1 bit) : Stop bit
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Rd/Wr (1 bit) : Read/Write bit. Rd equals 1, Wr equals 0.
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A, NA (1 bit) : Accept and reverse accept bit.
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Addr (7 bits): I2C 7 bit address. Note that this can be expanded as usual to
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get a 10 bit I2C address.
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Comm (8 bits): Command byte, a data byte which often selects a register on
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the device.
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Data (8 bits): A plain data byte. Sometimes, I write DataLow, DataHigh
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for 16 bit data.
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Count (8 bits): A data byte containing the length of a block operation.
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[..]: Data sent by I2C device, as opposed to data sent by the host adapter.
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Simple send transaction
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======================
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This corresponds to i2c_master_send.
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S Addr Wr [A] Data [A] Data [A] ... [A] Data [A] P
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Simple receive transaction
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===========================
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This corresponds to i2c_master_recv
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S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P
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Combined transactions
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====================
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This corresponds to i2c_transfer
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They are just like the above transactions, but instead of a stop bit P
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a start bit S is sent and the transaction continues. An example of
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a byte read, followed by a byte write:
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S Addr Rd [A] [Data] NA S Addr Wr [A] Data [A] P
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Modified transactions
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=====================
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The following modifications to the I2C protocol can also be generated by
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setting these flags for i2c messages. With the exception of I2C_M_NOSTART, they
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are usually only needed to work around device issues:
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I2C_M_IGNORE_NAK:
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Normally message is interrupted immediately if there is [NA] from the
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client. Setting this flag treats any [NA] as [A], and all of
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message is sent.
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These messages may still fail to SCL lo->hi timeout.
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I2C_M_NO_RD_ACK:
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In a read message, master A/NA bit is skipped.
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I2C_M_NOSTART:
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In a combined transaction, no 'S Addr Wr/Rd [A]' is generated at some
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point. For example, setting I2C_M_NOSTART on the second partial message
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generates something like:
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S Addr Rd [A] [Data] NA Data [A] P
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If you set the I2C_M_NOSTART variable for the first partial message,
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we do not generate Addr, but we do generate the startbit S. This will
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probably confuse all other clients on your bus, so don't try this.
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This is often used to gather transmits from multiple data buffers in
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system memory into something that appears as a single transfer to the
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I2C device but may also be used between direction changes by some
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rare devices.
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I2C_M_REV_DIR_ADDR:
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This toggles the Rd/Wr flag. That is, if you want to do a write, but
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need to emit an Rd instead of a Wr, or vice versa, you set this
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flag. For example:
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S Addr Rd [A] Data [A] Data [A] ... [A] Data [A] P
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I2C_M_STOP:
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Force a stop condition (P) after the message. Some I2C related protocols
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like SCCB require that. Normally, you really don't want to get interrupted
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between the messages of one transfer.
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