Servo v4

Servo v4 is a debug device in the Servo family.

Servo v4 functions as a configurable USB hub to support developer and lab recovery features. However, it doesn‘t have any hardware debug features on its own. It must be paired with CCD (Cr50’s on-board servo implementation) or Servo Micro.

Googlers: gLinux has a kernel bug that will cause all USB devices (including keyboard and mouse) to randomly stop working when using servo v4.

To enable tracing and help debug the issue, which also seems to help reduce the frequency of occurrence, you can add the following to /etc/rc.local:

logger "Enabling USB Tracing: See http://b/123886969 and http://b/136676682"
echo 1 > /sys/kernel/debug/tracing/events/xhci-hcd/enable

See this email for more details.

What is Servo v4?

Servo v4 combines the functionality of the following devices into one:

  • Ethernet-USB dongle
  • Muxable USB port
  • uSD to USB3.0 dongle
  • Keyboard emulator
  • Pass through charger
  • Case-Closed Debug (CCD) interface (SuzyQ debug cable)


Servo v4 with Accessories

Getting Servo v4

Sorry, Servo v4 is not publicly available for purchase.
IMPORTANT: You will need to update the firmware before using, as the factory firmware is quite old.


Your contact at Google should be able to provide you with Servo v4.


Stop by your local Chromestop.

How to Use Servo v4

Servo v4 can be plugged into a host machine using a uB USB cable. This will power Servo v4 while allowing the user to control Servo v4 using servod.

The DUT connector (which is a captive cable) can be plugged into a DUT, providing the DUT access to the ethernet and blue USB port.

The Type-C captive cable enables debugging of devices that have a Cr50 (recent Chromebooks) through CCD.

The “uServo” USB port can be used to plug a servo micro to debug devices over the Yoshi debug header.

From here other functionality is available. The following ports can be used to download data to a device:

  • Ethernet
  • uSD card
  • USB stick

Additionally Servo v4 can be used to power the DUT which becomes useful for devices that use USB as their only charge port (tablets, phones etc.). The Type-C port can be used to plug in any Type-C charger to provide full charging capabilities as a charge through hub. If no charger is plugged, Servo v4 will act as a passive hub.

Servo v4 has an embedded keyboard so keystrokes can be emulated on the DUT.

The servod server must be running for Servo v4 to work. Details can be found on the Servo page.

Type-A vs Type-C Variants

There are two variants of Servo V4: Type-A and Type-C.

Both versions use the same board and case, but have a different captive cable stuffed.

Type-C Version

The Type-C version acts as both a USB hub and PD charger. Servo v4 can also control both CC terminations which allows it to act as a debug accessory. It should be used on systems with CCD.

NOTE: Type-C Servo v4 is a charge-through hub and is NOT functionally equivalent to a Type-A servo with an A-to-C adapter. DO NOT use a Type-C Servo v4 just because you want to plug into a Type-C port. Older Chromebooks (Eve, Samus, etc.) have EC bugs that prevent charge through hubs from working correctly.

Servo v4 Type-C

Type-A Version

The Type-A version is used with a uServo and serves as the DUT USB hub, with the uServo providing SPI and UART support.

Servo v4 Type-A

Servo v4 Revisions

Servo v4 had several revisions, indicated by board color. The mass production (MP) version is available from Chromestop.

Green (MP)

Final version, has full functionality.

Blue (DVT)

The DVT version is completely functional except for the one bug affecting only Type-C variant. There are about one thousand of these, so you may encounter them.

  • PD Charge-through only works on chargers up to 10V.
  • Firmware identifies this by board revision and will not negotiate any voltages above 10V.

Red (EVT)

The EVT version is a mostly functioning version with a few bugs. There are around three hundred of these total.

  • The ethernet IC has a bug in it that makes the device fall out of GigE/USB3.0 speeds under certain circumstances and it falls down to USB1.1 / 2 speeds and never properly recovers until reset.
  • Type-C variant doesn't function, Type-A works OK.


Servo v4 runs more or less equivalently to Servo v2 and v3, through servod. It's intended to be mostly transparent, but there are some differences.

Most functionality is exported through dut-control.

(chroot) $ sudo servod -b <board> -s <serial>

To use with a specific board, you can connect a servo_micro to the “uServo” labeled port (or use the Type-C cable to connect to CCD) and run servod, which will load the board config and control both Servo v4 and Servo Micro (or Cr50).

(chroot) $ sudo servod -b [board] -s [serialno printed on servo v4 sticker]


Connect to Servo console without servod

Connect to Servo v4 Console:

(chroot) $ usb_console -d 18d1:501b

Connect to Servo Micro Console:

(chroot) $ usb_console -d 18d1:501a -i 3

Connect to Cr50 Console:

(chroot) $ usb_console -d 18d1:5014

Switch SD to Host

(chroot) $ dut-control sd_en:on sd_pwr_en:on sd_mux_sel:servo_sees_usbkey host_sd_usb_mux_en:on host_sd_usb_mux_sel:sd

Switch SD to DUT

(chroot) $ dut-control sd_en:on sd_pwr_en:on sd_mux_sel:dut_sees_usbkey

Disable SD

(chroot) $ dut-control sd_en:off sd_pwr_en:off

Switch USB3 to Host

(chroot) $ dut-control usb3_mux_en:on usb3_mux_sel:servo_sees_usbkey usb3_pwr_en:on host_sd_usb_mux_en:on host_sd_usb_mux_sel:usb

Switch USB3 to DUT

(chroot) $ dut-control usb3_mux_en:on usb3_mux_sel:dut_sees_usbkey usb3_pwr_en:on

Disable/Enable SuzyQ wiring (debug accessory mode)

Type-C Servo v4 only
(chroot) $ dut-control servo_v4_dts_mode:off [on]

Disable/Enable Chargethrough

Type-C Servo v4 only
(chroot) $ dut-control servo_v4_role:snk [src]


For CCD: Flashrom doesn't need to specify voltage anymore, this is done within Cr50. See the “care and feeding” for your specific device for the correct flashrom commands for CCD, Servo Micro, and Servo v2, as they are each different.
When flashing the BIOS or EC with CCD, you need to make sure the FlashAP capability is enabled in Cr50.

Flash BIOS with CCD:

(chroot) $ sudo flashrom -p raiden_debug_spi:target=AP -r bios.bin

Updating Firmware

The latest firmware is available in the chroot at /usr/share/servo_updater/firmware/.

If the green LED on the servo does not light up when DUT POWER is connected to a USB charger, you probably don’t have the latest firmware and should update.

NOTE: servod must not be running.

Sync the latest source:

(chroot) repo sync

Update sys-firmware/servo-firmware to the latest version:

(chroot) ~/trunk/src/scripts/update_chroot

Update to latest stable firmware:

(chroot) $ sudo servo_updater -b servo_v4

Update to specific version:

(chroot) $ sudo servo_updater -b servo_v4 -f <filename>

Enabling Case Closed Debug (CCD)

See CCD for complete details.

If using a Type-C Servo v4, these commands will only work if the USB-C cable is plugged into the correct USB port. Generally, this is the USB port on the left side of the device. If the command doesn't work, try the other ports.

Connect to Cr50 console:

(chroot) $ usb_console -d 18d1:5014

Check the Cr50 version in the Cr50 console:

> version
Build:   0.4.10/cr50_v1.9308_B.269-754117a

CCD requires Cr50 version 0.3.9+ / 0.4.9+

  • 0.4.x is the “pre-pvt” version, for pre-production devices.
  • 0.3.x is the “mp” version, for production devices. This requires the device to be in developer mode before “ccd open”.
  • 0.0.22 is the factory preflash from GUC. It needs an update.

Open CCD in the Cr50 console:

> ccd open

Press power button when prompted. It should take around 5 minutes.

If you get an access denied error when attempting ccd open, you likely do not have developer mode enabled.
Cr50 loses the developer mode state after “opening CCD”. If your device boots into recovery mode, try re-entering developer mode.

Enable testlab mode in the Cr50 console:

> ccd testlab enable

Press power button some more. CTRL+C to exit.

Run servod as normal, CCD should be enabled now.

Known issues

  • Displayport - not supported
  • Type-A/Type-C: Servo V4 comes in two variants on the captive cable, both can be obtained from Chromestop.
  • Type-C Servo is not interchangeable with Type-A. Type-C must be paired with CCD, Type-A must be paired with Servo Micro.
  • DFU: To use util/flash_ec you must enable DFU by connecting servo with an A-A cable plugged into an OTG cable. USB ID is tied to BOOT0 indicator on the STM32 part.

Interop Problems with Some Switches

See http://b/112187276.

Your network switch port will get hosed (some TX buffer fills up and starts dropping things) if you have Servo v4 connected to most switches and unplug the servo‘s USB connection, or power off the DUT fully, after the servo’s NIC has an IP. This behavior is not reproducible with a GS108Tv2, so the workaround is to use a GS108Tv2.


File bug or feature requests here.


You don‘t need to do this unless you’re developing Servo v4 firmware.

Servo v4 code lives in the EC and hdctools codebase. It can be built as follows:

(chroot) $ cd ~/trunk/src/platform/ec
(chroot) $ make BOARD=servo_v4 -j8

To raw flash a Servo v4, BOOT0 select pin is indicated by the OTG cable:

(chroot) $ ./util/flash_ec --board=servo_v4

To set the Servo v4 serial number on the Servo console:

> serialno set 012345