Minix i2c Driver
================
TODO: this probably belongs on the wiki
Overview
--------
This is the driver for the i2c bus. It provides the same /dev interface as
NetBSD and OpenBSD (see dev/i2c/i2c_io.h). It also provides an interface for
other drivers to access the I2C bus using Minix IPC.
Organization and Layout
-----------------------
i2c.c generic i2c bus driver
arch/ arch specific code
earm/ earm specific code
omap_i2c.c AM335X/DM37XX i2c bus driver
omap_i2c.h AM335X/DM37XX function prototypes
omap_i2c_registers.h AM335X/DM37XX register offsets, etc.
Testing the Code
----------------
Below are the steps needed to start up the i2c driver instances. Though,
now they get started at boot in /usr/etc/rc, it's still useful to know if
you take down the service and need to start it again.
Creating the device files (this is already done automatically, but if not):
cd /dev && MAKEDEV i2c-1 && MAKEDEV i2c-2 && MAKEDEV i2c-3
Starting up the instances:
/bin/service up /service/i2c -dev /dev/i2c-1 -label i2c.1 -args instance=1
/bin/service up /service/i2c -dev /dev/i2c-2 -label i2c.2 -args instance=2
/bin/service up /service/i2c -dev /dev/i2c-3 -label i2c.3 -args instance=3
There is an i2cscan program from NetBSD which can detect devices on the bus:
i2cscan -r /dev/i2c-1
i2cscan -r /dev/i2c-2
i2cscan -r /dev/i2c-3
Limitations
-----------
The i2c controllers used in the am335x and the dm37xx do not support zero
byte transfers. Writing 0 bytes is a common method used to probe the bus
for devices. Most of the address ranges i2cscan scans are done by this
method. Therefore, only a subset of devices on the bus will be detected by
i2cscan (i.e. the devices it detects using the 1 byte read method). See
the register description for I2C_CNT in the technical reference manuals
for details about why 0 byte transfers are not allowed.
Developing I2C Device Drivers
-----------------------------
The driver for the EEPROM (a.k.a. drivers/cat24c256) is the hello world of
Minix i2c device drivers. It shows how to use the i2cdriver library and
how to use the bus for reads and writes. commands/eepromread is another
place to look if you're interested in accessing devices through the /dev
interface.
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