Echo Dot 3rd Gen Digging Deeper

Following from the Echo Dot 3rd Gen Smart speaker Teardown post we have been exploring how the Echo Dot works and if it is possible to connect it to a computer to replace the firmware or install third party applications.

The first step was to make some high-resolution photographs of the PCBs to aid in analysing the inner workings of the Echo Dot. There are two PCBs inside the Echo Dot which we will call the base board and the top board. The designers at Amazon were very kind to include lots of test points on both PCBs which greatly aided in reverse engineering their functions.

Top Board

The top board contains the components for audio input, LED output and wireless communication.

The top side of the top board contains four push switches, 2 LEDs and a light sensor.

At the centre of the board is a MediaTek MT7658CSN dual-band Wi-Fi and Bluetooth controller. This controller is designed to be paired with the main CPU and communicates with the outside world.

There are 12 RGB LEDs around the outside edge of the board, these are controlled using an ISSI IS31FL3236 36 channel LED driver. The IS31FL3236 is controlled using an I2C bus.

Four microphones, part number UY19 2W07, connect to two Texas Instruments TLV320ADC3101 ADCs. The TLV320ADC3101 is a stereo ADC designed for audio applications and contains a built in DSP. The TLV320ADC3101 communicates with the CPU using an I2C and I2S bus. I2C is used to configure the ADCs on start-up and the audio stream is sent along the I2S bus.

There is a Fairchild 74LCX74 dual Flip-Flop on the PCB. When we wrote the original teardown blog post we speculated that the 74LCX74 is used for disabling the microphones. This was partially confirmed as driving pin 11 (Clock Pulse Input 2) high turns on the red LED ring to indicate that the microphone is disabled.

How the Echo goes about disabling the microphones themselves is still unknown as the ADCs do not contain an enable pin and we could not find any way of disabling the microphones electronically so the control of the microphones is most likely software based which opens up the possibility of listening to the microphones even when the user believes they are disabled.

There are three other ICs on the board marked UY96O, L12 and DJ4. Their purpose is currently unknown.

A 39-pin flat flex cable joins the top board to the base board. The cable carries data and power between the two boards. Most of the pins on the cable have corresponding test points so with a fast-enough logic analyser it should be possible to monitor the communication between the CPU and wireless controller.

There are several ground tags which connect to the metal chassis when the board is screwed into place.

The top board contains 89 test points. After lots of probing with a multimeter and oscilloscope we were able to determine the purpose of many of the test points. The table below shows a list of the test points and their purpose.

Logic Analyser connected to the test pins.

Two of the test points TM7 and TM11 connect to the I2C pins on the ADC U2 so we connected them to a logic analyser to see what data is being sent down the I2C bus.

The CPU tries to communicate with five devices on the I2C bus.

0x30 and 0x34 are the two ADC chips.

0x72 is unknown but a guess would be the light sensor on the top of the board as this is the only device which is read from as well as written to.

0x7E is the LED driver. The CPU sends a constant stream of packets to the LED driver while the device is powered.

0x88 The CPU sends a write packet to this address about 5 seconds into the boot process but does not receive a reply so this could be a device which was omitted from the final design, but the firmware was not updated to reflect this.

Base Board

The base board contains the main CPU, a Mediatek MT8516BAAA. We couldn’t find much about the MT8516 apart from a few short notes on Mediateks website. What we do know is that it contains a quad-core, 64-bit ARM® Cortex-A35 core which operates at up to 1.3 GHz.

Next to the CPU is a Samsung KMFN60012M-B214 which contains 8Gb of eMMC flash memory and 1Gb of LPDDR3 ram.

A Texas Instruments TAS2770 audio driver provides the amplification for the internal speaker. The speaker connects to the base board via two sprung pins.

The rest of the base board is dedicated to power regulation with several linear and switching regulators.

The back of the base board contains 28 test points. Most of these are dedicated to the outputs from the voltage regulators but two test points caught our attention.

In the bottom of the plastic enclosure is a small rectangular hole with 6 test points visible on the base board. Four of the test points are related to power with a 12V, 1.9V and two ground pins. Two of the test points TM4 and TM5 connect to a pair of traces that looked suspiciously like a differential pair and probing them showed that TM4 was 0V and TM5 was 3.49V. These are the voltages found on the data lines of a USB cable and previous versions of the Echo have included a USB port for firmware updates, so it stood to reason that these two pins were connected to a USB interface.

The test points are spaced on a 2mm 3x2 grid so we soldered a 2mm pitched connector onto the test pins and designed a small PCB which would connect the pins to a micro USB socket. As there was already a hole in the base of the case, we could connect the new USB interface to the Echo Dot when it is assembled. When we designed the PCB we were not 100% sure whether the USB traces were the correct way around or if the 1.9V pin was used for anything else so we added a row of 0R resistors between the USB socket and the header pins so we would be able to link the pins in any combination with jumper wires if necessary.

After waiting several weeks for the PCB to arrive from China we assembled the new USB adapter and connected it to an old sacrificial netbook that we keep around for plugging in unknow USB devices. If we got something wrong, we didn’t want to destroy the USB interfaces on our workstations.

The new debug header to USB board.

Debug adapter fitted to the base of the Echo Dot with extra padded feet stuck onto the case to give clearance for the PCB.

Running the linux command “lsusb -v” gave us a list of USB devices connected to the netbook and in the list was a Mediatek device with the following details.

Bus 001 Device 006: ID 0e8d:2008 MediaTek Inc. 
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               2.00
  bDeviceClass            0 
  bDeviceSubClass         0 
  bDeviceProtocol         0 
  bMaxPacketSize0        64
  idVendor           0x0e8d MediaTek Inc.
  idProduct          0x2008 
  bcdDevice           ff.ff
  iManufacturer           3 MediaTek
  iProduct                4 AEODN
  iSerial                 5 XXXXXXXXXXXX
  bNumConfigurations      1
  Configuration Descriptor:
    bLength                 9
    bDescriptorType         2
    wTotalLength       0x0027
    bNumInterfaces          1
    bConfigurationValue     1
    iConfiguration          0 
    bmAttributes         0xc0
      Self Powered
    MaxPower              500mA
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        0
      bAlternateSetting       0
      bNumEndpoints           3
      bInterfaceClass       255 Vendor Specific Class
      bInterfaceSubClass    255 Vendor Specific Subclass
      bInterfaceProtocol      0 
      iInterface             17 MTP
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x81  EP 1 IN
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval               0
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x01  EP 1 OUT
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval               0
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x82  EP 2 IN
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x001c  1x 28 bytes
        bInterval               6
Device Qualifier (for other device speed):
  bLength                10
  bDescriptorType         6
  bcdUSB               2.00
  bDeviceClass            0 
  bDeviceSubClass         0 
  bDeviceProtocol         0 
  bMaxPacketSize0        64
  bNumConfigurations      1
Device Status:     0x0001
  Self Powered

As suspected the test points are a USB interface for flashing the firmware on the Echo Dot. We found some software for flashing MediaTek devices but quickly realised that the software needs a scatter file which is similar to a partition table and contains the information needed for the software to read and write to the MediaTek processor.

Searches online for a scatter file for an MT8516 gave us no results but we did find a reference to booting the Echo into fastboot mode by holding down one of the buttons on startup.  After a bit of experimenting we found that holding down the button with the white dot while turning on the device boots the Echo into fastboot mode and turns the LED ring green.

Running the command "fastboot getvar all" returned the following:

root@echodev:~$ fastboot getvar all > fastboot.txt
(bootloader) 	antirback_tee_version: 0x0001
(bootloader) 	antirback_lk_version: 0x0001
(bootloader) 	antirback_pl_version: 0x0001
(bootloader) 	rpmb_state: 1
(bootloader) 	lk_build_desc: 09a5edd-20190510_065840
(bootloader) 	pl_build_desc: 2b7c882-20190510_065840
(bootloader) 	secure: yes
(bootloader) 	prod: 1
(bootloader) 	unlock_status: false
(bootloader) 	tu_code: q/3z6mhb4gVE0G9ed5H6QkQrlEwMEdyiotGFiQJa/wM=
(bootloader) 	unlock_code: xxxxxxxxxxxxx
(bootloader) 	serialno: xxxxxxxxxxxxxx
(bootloader) 	max-download-size: 0x8000000
(bootloader) 	kernel: lk
(bootloader) 	product: DONUT
(bootloader) 	version-preloader: 0.1.00
(bootloader) 	version: 0.5
all: Done!

Fastboot can see the device but as the bootloader is locked we can not extract or update the firmware image, so for the moment we have come to a dead end. Hopefully someone else will have more luck with accessing the Echo Dots firmware.

PCB Files

You can download the USB adapter PCB files in Diptrace format and also the gerber files from this link.

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