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- ================
- The I2C Protocol
- ================
- This document is an overview of the basic I2C transactions and the kernel
- APIs to perform them.
- Key to symbols
- ==============
- =============== =============================================================
- S Start condition
- P Stop condition
- Rd/Wr (1 bit) Read/Write bit. Rd equals 1, Wr equals 0.
- A, NA (1 bit) Acknowledge (ACK) and Not Acknowledge (NACK) bit
- Addr (7 bits) I2C 7 bit address. Note that this can be expanded to
- get a 10 bit I2C address.
- Data (8 bits) A plain data byte.
- [..] Data sent by I2C device, as opposed to data sent by the
- host adapter.
- =============== =============================================================
- Simple send transaction
- =======================
- Implemented by i2c_master_send()::
- S Addr Wr [A] Data [A] Data [A] ... [A] Data [A] P
- Simple receive transaction
- ==========================
- Implemented by i2c_master_recv()::
- S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P
- Combined transactions
- =====================
- Implemented by i2c_transfer().
- They are just like the above transactions, but instead of a stop
- condition P a start condition 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 by
- setting these flags for I2C messages. With the exception of I2C_M_NOSTART, they
- are usually only needed to work around device issues:
- 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.
- I2C_M_NO_RD_ACK:
- In a read message, master A/NA bit is skipped.
- 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 start condition 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.
- 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
- I2C_M_STOP:
- Force a stop condition (P) after the message. Some I2C related protocols
- like SCCB require that. Normally, you really don't want to get interrupted
- between the messages of one transfer.
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