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