Intel Galileo: How to control the I/O expander via command line


Note: This tutorial is for the Galileo Gen 1 only.

You may be aware that the Galileo uses an I/O expander chip to add additional pins onto the Quark X1000. This chip, the Cypress CY8C9540A, effectively multiplexes the majority of the Galileo's GPIO pins and acts as a PWM controller. For the most part, its behavior is completely transparent -- but there are certain situations where it may be helpful to directly control it.

To control the CY8C9540A from the command line, we'll use the Linux utilities i2cget and i2cset. You'll need the Intel IoT Dev Kit image to use these, although you can also compile your own Yocto build as necessary.

The CY8C9540A will have an address of either 0x20 or 0x21, depending on the position of jumper J2. When the jumper is over the pin with the white triangle next to it, the address should be 0x20. You can also check this by running the command ls /sys/bus/i2c/devices, where either 0-0020 or 0-0021 should appear.

The first thing we need to do is disable the image's kernel module for controlling the expander -- this can be done by running the command rmmod cy8c9540a. If not, the device will constantly appear as "busy", because the Galileo will constantly spam it with commands so that it matches the state of everything in sysfs.

The part's datasheet will be needed to figure out the I2C interface. The easiest thing to do right away is to check the Device ID/Status Register located at 0x2E. Use the following command:

i2cget 0 0x20 0x2e b

The "0" refers to the desired I2C bus -- there's only on the Galileo, so it gets the number zero. "0x20" refers to the device address. "0x2e" refers to the desired register to be read. And "b" indicates that one byte should be read at a time. You may find it convenient to add the -y flag to avoid being prompted every time with the warning message about messing around on the I2C bus.

This command should return 0x40 if everything is working properly. The first nibble is the device family (4) and the second nibble is 0 or 1, depending on whether factory settings or user settings are applied. By default, the Galileo applies its own user settings.

The following command resets the chip to factory defaults (don't worry -- the Galileo restores its custom settings on boot):

i2cset 0 0x20 0x30 0x02 b

This writes the value 0x02 to the command register at 0x30. There are many other useful applications of this technique, including changing the drive mode, pullup settings, and PWM frequency/duty cycle. Many of these settings are not directly exposed through sysfs, such as open drain and open source drive modes.

To reload the normal kernel module, just run:

modprobe cy8c9540a

You can check that it's working using lsmod, where an entry for cy8c9540a should appear.