Servo Micro

Servo Micro (aka “uServo”) is a self contained replacement for Yoshi Servo Flex. It is meant to be compatible with Servo v2/v3 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.

Overview

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

• Servo Micro Schematics
• Case Closed Debug with Servo Micro

Use

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

(chroot) $ sudo servod -b [board]

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.

Flashing the BIOS or EC with Servo Micro

Servo Micro can be used to flash the AP BIOS or EC with flashrom. Note that flashrom's command line needs to specify the correct endpoint, which is raiden rather than ftdi.

For example, on a 3.3V DUT:

(chroot) $ dut-control spi2_vref:pp3300 spi2_buf_en:on
(chroot) $ sudo flashrom --programmer raiden_debug_spi -r bios.bin
(chroot) $ dut-control spi2_vref:off spi2_buf_en:off

NOTE: There is no target=(AP|EC) command as with other raiden_debug_spi devices. Adding this flag to the flashrom command will cause an error.

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.

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) ~/trunk/src/scripts/update_chroot

Update the firmware:

(chroot) $ sudo servo_updater -b servo_micro

Developing Servo Micro Firmware

Compiling

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

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

Flashing

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 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.