Servo Micro (aka “uServo”) is a self contained replacement for Yoshi Servo Flex. It is meant to be compatible with Servo v2 via servod. The design uses Case Closed Debug software on an STM32 microcontroller to provide a CCD interface into systems with a Yoshi debug port.


Servo Micro is usually paired with a Servo v4 Type-A, which provides ethernet, dut hub, and muxed usb storage.

Servo Micro


Like other Servo boards, the Servo Micro requires servod to be running:

(chroot) $ sudo servod -b [board]

The Servo Micro connects to the servo header of the DUT in the orientation where ‘dot’ indications from the Servo Micro and the servo header align:

Servo Micro and Header Dot Close-up

Most Servo header functionality should be available, used in the same way as any other servo. Here you can see the Servo v4 and Servo Micro plugged into a system.

Servo Micro and Servo v4

Flashing the BIOS or EC with Servo Micro

Reading or flashing AP firmware (BIOS) or EC firmware with Servo Micro is done with the same commands as any servo type. Under the hood the implementation for each servo type can vary, but that is abstracted away from you as a user.

For example, to read and flash AP firmware (BIOS):

(chroot) $ sudo futility read --servo -v "$OUTFILE"
(chroot) $ sudo futility update --servo -v -i "$INFILE"

Servo Micro Header Pins

The names of the pins loosely correspond to the name of the controls used by servod. For example, DUT_LID_OPEN corresponds to lid_open. See the Servo Micro Schematics for complete details of how the header is connected to the Servo Micro internals.

Servo Micro Header Pins

Power Sourcing

Servo Micro is designed to supply power to power-monitoring ICs, independent from the DUT's power supply. This allows more accurate measurements of total system power when the DUT is in a low-power state. We expect less than 50mW of power if the maximum number of power-monitoring IC is in use drawing from the PP3300 pin.

The PP3300 and VREF pins on the connector can provide a maximum total of 100mA of current before exceeding limits and damaging the regulators. Future designs may reduce the limit and use resettable fuses to reduce the risk of damaging components. Designs should target a limit under 50mA.

Known Issues

  • JTAG - Servo Micro does not support JTAG.
  • Servo Micro doesn't support jtag_vref, or any _buf_on_flex controls.
  • If a Servo Micro is plugged into a DUT without being plugged into a host machine, it can cause undefined behavior on the DUT.

Uart Failures


On Servo micro (and perhaps other STM based Servos (v4)) the UART can become inoperable and require the USB port to be reset to recover.

This will manifest itself as failures like:

Servod - ERROR - Problem initializing SERVO_JTAG_TDI -> off


Problem with ['servo_v4_version'] :: Timeout waiting for response

Overcurrent and Brownouts


Some device configurations (verified on cyan boards) are able to draw excessive current from the Servo Micro, putting the hardware in a non-responsive state. Brownout detection has been enabled to escape this condition by rebooting the servos impacted. Impacted DUTs may require deployment using Servo v2.

Updating Firmware

Servo Micro can be updated to the latest stable firmware using the servo_updater tool.

NOTE: servod must not be running when updating since it locks the device.

Sync the latest source:

(chroot) repo sync

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

(chroot) update_chroot

Update the firmware:

(chroot) $ sudo servo_updater -b servo_micro

Developing Servo Micro Firmware


servo micro code lives in the ec codebase. It can be built as follows:

(chroot) $ cd ~/chromiumos/src/platform/ec
(chroot) $ make BOARD=servo_micro -j8


To flash a working Servo Micro:

(chroot) $ sudo servo_updater -b servo_micro -f build/servo_micro/ec.bin

If the flash is empty or the image is broken, you can flash the Servo Micro using the BOOT0 select pin, which is held by USB OTG's ID pin. The easiest way to do this is to plug a USB OTG adapter into Servo Micro, then plug an A-A cable into your desktop and Servo Micro.

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

Servo Micro USB OTG

Servo Micro UART

Servo Micro exports a console UART over the small header next to the USM-micro port. There is a rare cable for this header, but it's easier to just rework UART wires on and use a standard FTDI pin header 3.3V cable.

Servo Micro UART

Servo Micro As GSC Console

Sometimes it is necessary to debug GSC interactions with the DUT, especially during early hardware bringup when GSC is still not communicating reliably over USB.

This is when the direct access to the GSC console is needed. In this situation GSC console could be accessed over a dedicated device created by servod.

But having to run servod (and create the Chrome OS chroot for that) is a pretty high barrier for many users. Luckily Servo Micro provides all necessary features directly, no need to create chroot and run servod to access them.

To use Servo Micro as the GSC console, attach Servo Micro to the servo header on the DUT and connect it to the workstation as described above.

The following command allows you to find tty USB devices created to access various consoles available through Servo Micro:

$ find /dev -type l -name '*Servo_Micro*' -exec readlink -f "{}" +

If you don't see the four devices unplug and re-plug the USB cable connecting to Servo Micro.

Of the four /dev/ttyUSBx devices, the lowest index device is the GSC console, and the second lowest index is the Servo Micro console.

To get access to GSC console, first connect to the Servo Micro console and send it commands to completely disengage Servo Micro from the DUT, leaving only GSC console connected:

$  minicom -D /dev/ttyUSB<second lowest>
> gpioset UART2_EN_L 1         # Disconnect the EC UART
> gpioset UART1_EN_L 1         # Disconnect the AP UART
> i2cxfer w 0 0x20 6 0xff      # Set all extended GPIOs as inputs
> i2cxfer w 0 0x20 7 0xff
> i2cxfer w 0 0x20 3 0         # Prepare GSC reset output level

To connect to the GSC console run minicom -D /dev/ttyUSB<lowest>.

To reset the GSC, issue the following two commands on the Servo Micro console

> i2cxfer w 0 0x20 7 0xbf     # Assert reset
> i2cxfer w 0 0x20 7 0xff     # Deassert reset