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