1da177e4c3
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
2.4 KiB
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
We have found some I2C devices that needs the following modifications:
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.
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<61>[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.