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88 lines
3.0 KiB
88 lines
3.0 KiB
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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