Enabling the Serial Port on a Nexus One

The Nexus One USB port works as a serial port when there's no power coming into the connector. But the stock kernel sets it up as a console. To use it as a general-purpose serial port, you have to use a kernel with CONFIG_USB_ANDROID_ACM enabled and CONFIG_MSM_SERIAL_DEBUGGER disabled.

If you want to skip installing the NDK, downloading and compiling the kernel, you can try this precompiled zImage.

We did this on a rooted Nexus One with an unlocked bootloader running Cyanogen CM6. It's probably not necessary to be running Cyanogen for this to work.

Download NDK from here:
http://developer.android.com/sdk/ndk/index.html

Install NDK using instructions from the website.

Make sure arm-eabi-gcc is in your path. It's in android-ndk-r4b/build/prebuilt/linux-x86/arm-eabi-4.4.0/bin.

(export PATH=$PATH:/path/to/android-ndk-r4b/build/prebuilt/linux-x86/arm-eabi-4.4.0/bin to add it)

Make sure your machine has git: $ sudo apt-get install git-core

$ git clone git://android.git.kernel.org/kernel/msm.git
$ git branch -a
$ git checkout -b android-msm-2.6.32 origin/android-msm-2.6.32
$ cd msm
$ ARCH=arm CROSS_COMPILE=arm-eabi- make mahimahi_defconfig
$ ARCH=arm CROSS_COMPILE=arm-eabi- make menuconfig
Exit the menu and say "yes" when it asks if you want to save.

$ gedit .config
Turn on CONFIG_USB_ANDROID_ACM (uncomment it and change "is not set" for "=y").
Turn off CONFIG_MSM_SERIAL_DEBUGGER (comment it out)
Turn off CONFIG_MSM_FIQ_SUPPORT
$ ARCH=arm CROSS_COMPILE=arm-eabi- make

The kernel image should compile and is found in arch/arm/boot/zImage

Try kernel without permanently storing it on the phone. Download fastboot from here:
http://developer.htc.com/adp.html

Reboot into fastboot mode. Power up the Nexus One while holding the trackball. Plug in the USB cable and the fastboot screen should say "fastboot usb".

$ sudo ./fastboot boot /path/to/msm/arch/arm/boot/zImage

If you get this message, then you probably forgot to run as root.

sm/arch/arm/boot/zImage
creating boot image...
creating boot image - 2232320 bytes
< waiting for device >

Once the phone boots up, use adb shell to ls /dev/tty* and if you see dev/ttyMSM0, it worked. (We haven't tried actually using the port yet).

When you reboot you'll be back into your old kernel. Haven't flashed this for real yet.

Check out the xda-developers.com thread linked below for more info on actually using the serial port.

The following sites were helpful:
http://androidboss.com/custom-linux-kernel-for-nexus-one/
http://forum.xda-developers.com/archive/index.php/t-625434.html