Two Wire Interface in AVR DA/DB

Two Wire Interface is AVR peripheral capable of supporting both I²C and SMBus.

Usage

Pointer to initialized struct i2c_master_s can be obtained using

FAR struct i2c_master_s *i2c;
i2c = avrdx_initialize_twi(0);

This function will initialize the peripheral based on configuration specified via Kconfig. It may be called multiple times, only first call will perform the initialization - subsequent calls will only return the pointer.

Deinitialization is currently not supported.

The parameter denotes which peripheral is to be initialized. With current chips, permitted values are 0 and 1. It is ignored if the chip has only single TWI interface.

Configuration options

Pin selection

This option makes it possible to choose which I/O pins will be connected to the peripheral.

Mode selection

The peripheral supports Standard, Fast and Fast Plus modes.

Quick commands

SMBus permits transactions that contain no data. Instead, the R/nW bit is interpreted as a single-bit datum by the target device. If this is enabled, I²C messages with zero length are interpreted as quick commands. If not, zero-length messages are not permitted and the transmission is rejected with EINVAL

Forbid NOSTART

NuttX upper half of the I²C driver permits I2C_M_NOSTART flag for a message, indicating that the message is not a standalone entity but rather a continuation of previous message. Since not all drivers need this and program memory space is not unlimited on the chip, this configuration option can be used to remove support for such messages.

Transmission that submits message with this flag will be rejected with EINVAL if this configuration option is set. Enable this only if you know internals of I²C drivers used by your application.

Limitations

Mode and addressing

Currently, only master mode is supported.

Only 7-bit addressing is supported. The peripheral does not support 10-bit addressing directly and the driver software does currently have no support for that either. I²C messages requesting 10-bit address are rejected with ENOTSUP.

Message limitations

Maximum message count for single transmission is 127.

The driver does not support I2C_M_NOSTOP flag for last submitted message. As per rules specified in include/nuttx/i2c/i2c_master.h, the I²C driver lower half is supposed to leave the bus occupied if the last of the submitted messages has this NOSTOP flag set. This driver is not capable of doing that.

Also not supported is I2C_M_NOSTART flag for first submitted message. First message within the transfer always starts with START condition.

Messages with zero length (which are only permitted based on configuration - see above) may not specify I2C_M_NOSTART nor I2C_M_NOSTOP. (Such a message is interpreted as a quick command and quick commands must end with a STOP condition according to the chip’s documentation.)

Inactivity timeout

The driver configures timeout for bus inactivity during initialization. This timeout is used by the hardware to switch its internal state from “bus is busy” or “bus is in unknown state” to “bus is idle.” This transition otherwise only happens after a STOP condition is detected or when forced by software.

This timeout remains in effect during normal TWI operation as well. Such a timeout is defined for SMBus but not for I²C; nevertheless, the driver does not distinguish between the two and the timeout is always in place.