Several of the Sipeed Maix series of boards cotain the Kendryte K210 processor, a 64-bit RISC-V CPU. This processor contains several peripherals to accelerate neural network processing and other “ai” tasks. This includes a “KPU” neural network processor, an audio processor supporting beamforming reception, and a digital video port supporting capture and output at VGA resolution. Other peripherals include 8M of SRAM (accessible with and without caching); remappable pins, including 40 GPIOs; AES, FFT, and SHA256 accelerators; a DMA controller; and I2C, I2S, and SPI controllers. Maix peripherals vary, but include spi flash; on-board usb-serial bridges; ports for cameras, displays, and sd cards; and ESP32 chips.

Currently, only the Sipeed MAIX BiT V2.0 (bitm) and Sipeed MAIXDUINO are supported, but the boards are fairly similar.

Documentation for Maix boards is available from Sipeed’s website. Documentation for the Kendryte K210 is available from Kendryte’s website. However, hardware details are rather lacking, so most technical reference has been taken from the standalone sdk.

Build and boot steps

To build U-Boot, run

make <defconfig>
make CROSS_COMPILE=<your cross compile prefix>

To flash U-Boot, run

kflash -tp /dev/<your tty here> -B <board_id> u-boot-dtb.bin

The board provides two serial devices, e.g.

  • /dev/serial/by-id/usb-Kongou_Hikari_Sipeed-Debug_12345678AB-if00-port0
  • /dev/serial/by-id/usb-Kongou_Hikari_Sipeed-Debug_12345678AB-if01-port0

Which one is used for flashing depends on the board.

Currently only a small subset of the board features are supported. So we can use the same default configuration and device tree. In the long run we may need separate settings.

Board defconfig board_id TTY device
Sipeed MAIX BiT sipeed_maix_bitm_defconfig bit first
Sipeed MAIX BiT with Mic sipeed_maix_bitm_defconfig bit_mic first
Sipeed MAIXDUINO sipeed_maix_bitm_defconfig maixduino first
Sipeed MAIX GO   goE second
Sipeed MAIX ONE DOCK   dan first

Flashing causes a reboot of the device. Parameter -t specifies that the serial console shall be opened immediately. Boot output should look like the following:

U-Boot 2020.04-rc2-00087-g2221cc09c1-dirty (Feb 28 2020 - 13:53:09 -0500)

DRAM:  8 MiB
MMC:   spi@53000000:slot@0: 0
In:    serial@38000000
Out:   serial@38000000
Err:   serial@38000000


OpenSBI is an open source supervisor execution environment implementing the RISC-V Supervisor Binary Interface Specification [1]. One of its features is to intercept run-time exceptions, e.g. for unaligned access or illegal instructions, and to emulate the failing instructions.

The OpenSBI source can be downloaded via:

git clone

As OpenSBI will be loaded at 0x80000000 we have to adjust the U-Boot text base. Furthermore we have to enable building U-Boot for S-mode:


Both settings are contained in sipeed_maix_smode_defconfig so we can build U-Boot with:

make sipeed_maix_smode_defconfig

To build OpenSBI with U-Boot as a payload:

cd opensbi
make \
PLATFORM=kendryte/k210 \
FW_PAYLOAD_PATH=<path to U-Boot>/u-boot-dtb.bin

The value of FW_PAYLOAD_OFFSET must match CONFIG_SYS_TEXT_BASE - 0x80000000.

The file to flash is build/platform/kendryte/k210/firmware/fw_payload.bin.


The default boot process is to load and boot the files /uImage and /k210.dtb off of the first partition of the MMC. For Linux, this will result in an output like

U-Boot 2020.10-00691-gd1d651d988-dirty (Oct 16 2020 - 17:05:24 -0400)

DRAM:  8 MiB
MMC:   spi@53000000:slot@0: 0
Loading Environment from SPIFlash... SF: Detected w25q128fw with page size 256 Bytes, erase size 4 KiB, total 16 MiB
In:    serial@38000000
Out:   serial@38000000
Err:   serial@38000000
Hit any key to stop autoboot:  0
1827380 bytes read in 1044 ms (1.7 MiB/s)
13428 bytes read in 10 ms (1.3 MiB/s)
## Booting kernel from Legacy Image at 80060000 ...
   Image Name:   linux
   Image Type:   RISC-V Linux Kernel Image (uncompressed)
   Data Size:    1827316 Bytes = 1.7 MiB
   Load Address: 80000000
   Entry Point:  80000000
   Verifying Checksum ... OK
## Flattened Device Tree blob at 80400000
   Booting using the fdt blob at 0x80400000
   Loading Kernel Image
   Loading Device Tree to 00000000803f9000, end 00000000803ff473 ... OK

Starting kernel ...

[    0.000000] Linux version 5.9.0-00021-g6dcc2f0814c6-dirty (sean@godwin) (riscv64-linux-gnu-gcc (GCC) 10.2.0, GNU ld (GNU Binutils) 2.35) #34 SMP Fri Oct 16 14:40:57 EDT 2020
[    0.000000] earlycon: sifive0 at MMIO 0x0000000038000000 (options '115200n8')
[    0.000000] printk: bootconsole [sifive0] enabled
[    0.000000] Zone ranges:
[    0.000000]   DMA32    [mem 0x0000000080000000-0x00000000807fffff]
[    0.000000]   Normal   empty
[    0.000000] Movable zone start for each node
[    0.000000] Early memory node ranges
[    0.000000]   node   0: [mem 0x0000000080000000-0x00000000807fffff]
[    0.000000] Initmem setup node 0 [mem 0x0000000080000000-0x00000000807fffff]
[    0.000000] riscv: ISA extensions acdfgim
[    0.000000] riscv: ELF capabilities acdfim
[    0.000000] percpu: max_distance=0x18000 too large for vmalloc space 0x0
[    0.000000] percpu: Embedded 12 pages/cpu s18848 r0 d30304 u49152
[    0.000000] Built 1 zonelists, mobility grouping off.  Total pages: 2020
[    0.000000] Kernel command line: earlycon console=ttySIF0
[    0.000000] Dentry cache hash table entries: 1024 (order: 1, 8192 bytes, linear)
[    0.000000] Inode-cache hash table entries: 512 (order: 0, 4096 bytes, linear)
[    0.000000] Sorting __ex_table...
[    0.000000] mem auto-init: stack:off, heap alloc:off, heap free:off
[    0.000000] Memory: 6004K/8192K available (1139K kernel code, 126K rwdata, 198K rodata, 90K init, 81K bss, 2188K reserved, 0K cma-reserved)
[    0.000000] rcu: Hierarchical RCU implementation.
[    0.000000] rcu: RCU calculated value of scheduler-enlistment delay is 25 jiffies.
[    0.000000] NR_IRQS: 64, nr_irqs: 64, preallocated irqs: 0
[    0.000000] riscv-intc: 64 local interrupts mapped
[    0.000000] plic: interrupt-controller@C000000: mapped 65 interrupts with 2 handlers for 2 contexts.
[    0.000000] random: get_random_bytes called from 0x00000000800019a8 with crng_init=0
[    0.000000] k210-clk: clock-controller
[    0.000000] k210-clk: clock-controller: fixed-rate 26 MHz osc base clock
[    0.000000] clint: clint@2000000: timer running at 7800000 Hz
[    0.000000] clocksource: clint_clocksource: mask: 0xffffffffffffffff max_cycles: 0x3990be68b, max_idle_ns: 881590404272 ns
[    0.000014] sched_clock: 64 bits at 7MHz, resolution 128ns, wraps every 4398046511054ns
[    0.008450] Console: colour dummy device 80x25
[    0.012494] Calibrating delay loop (skipped), value calculated using timer frequency.. 15.60 BogoMIPS (lpj=31200)
[    0.022693] pid_max: default: 4096 minimum: 301
[    0.027352] Mount-cache hash table entries: 512 (order: 0, 4096 bytes, linear)
[    0.034428] Mountpoint-cache hash table entries: 512 (order: 0, 4096 bytes, linear)
[    0.045099] rcu: Hierarchical SRCU implementation.
[    0.050048] smp: Bringing up secondary CPUs ...
[    0.055417] smp: Brought up 1 node, 2 CPUs
[    0.059602] devtmpfs: initialized
[    0.082796] clocksource: jiffies: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 7645041785100000 ns
[    0.091820] futex hash table entries: 16 (order: -2, 1024 bytes, linear)
[    0.098507] pinctrl core: initialized pinctrl subsystem
[    0.140938] clocksource: Switched to clocksource clint_clocksource
[    0.247216] workingset: timestamp_bits=62 max_order=11 bucket_order=0
[    0.277392] k210-fpioa 502b0000.pinmux: K210 FPIOA pin controller
[    0.291724] k210-sysctl 50440000.syscon: K210 system controller
[    0.305317] k210-rst 50440000.syscon:reset-controller: K210 reset controller
[    0.313808] 38000000.serial: ttySIF0 at MMIO 0x38000000 (irq = 1, base_baud = 115200) is a SiFive UART v0
[    0.322712] printk: console [ttySIF0] enabled
[    0.322712] printk: console [ttySIF0] enabled
[    0.331328] printk: bootconsole [sifive0] disabled
[    0.331328] printk: bootconsole [sifive0] disabled
[    0.353347] Freeing unused kernel memory: 88K
[    0.357004] This architecture does not have kernel memory protection.
[    0.363397] Run /init as init process

Loading, Booting, and Storing Images

Loading Images


Use the loady command to load images over serial.

=> loady $loadaddr 1500000
## Switch baudrate to 1500000 bps and press ENTER ...

*** baud: 1500000

*** baud: 1500000 ***
## Ready for binary (ymodem) download to 0x80000000 at 1500000 bps...
*** file: loader.bin
$ sz -vv loader.bin
Sending: loader.bin
Bytes Sent:2478208   BPS:72937
Ymodem sectors/kbytes sent:   0/ 0k
Transfer complete

*** exit status: 0 ***
## Total Size      = 0x0025d052 = 2478162 Bytes
## Switch baudrate to 115200 bps and press ESC ...

*** baud: 115200

*** baud: 115200 ***

This command does not set $filesize, so it may need to be set manually.

SPI Flash

To load an image off of SPI flash, first set up a partition as described in Partitions. Then, use mtd to load that partition

=> sf probe
SF: Detected w25q128fw with page size 256 Bytes, erase size 4 KiB, total 16 MiB
=> mtd read linux $loadaddr
Reading 2097152 byte(s) at offset 0x00000000

This command does not set $filesize, so it may need to be set manually.


The MMC device number is 0. To list partitions on the device, use part:

=> part list mmc 0

Partition Map for MMC device 0  --   Partition Type: EFI

Part    Start LBA       End LBA          Name
        Type GUID
        Partition GUID
  1     0x00000800      0x039effde      "boot"
        attrs:  0x0000000000000000
        type:   c12a7328-f81f-11d2-ba4b-00a0c93ec93b
        guid:   96161f7d-7113-4cc7-9a24-08ab7fc5cb72

To list files, use ls:

=> ls mmc 0:1
<DIR>       4096 .
<DIR>       4096 ..
<DIR>      16384 lost+found
           13428 k210.dtb
         1827380 uImage

To load a file, use load:

=> load mmc 0:1 $loadaddr uImage
1827380 bytes read in 1049 ms (1.7 MiB/s)

Running Programs


To run a bare binary, use the go command:

=> go 80000000
## Starting application at 0x80000000 ...
Example expects ABI version 9
Actual U-Boot ABI version 9
Hello World
argc = 1
argv[0] = "80000000"
argv[1] = "<NULL>"
Hit any key to exit ...

Note that this will only start a program on one hart. As-of this writing it is only possible to start a program on multiple harts using the bootm command.

Legacy Images

To create a legacy image, use tools/mkimage:

$ tools/mkimage -A riscv -O linux -T kernel -C none -a 0x80000000 -e 0x80000000 -n linux -d ../linux-git/arch/riscv/boot/Image uImage
Image Name:   linux
Created:      Fri Oct 16 17:36:32 2020
Image Type:   RISC-V Linux Kernel Image (uncompressed)
Data Size:    1827316 Bytes = 1784.49 KiB = 1.74 MiB
Load Address: 80000000
Entry Point:  80000000

The bootm command also requires an FDT, even if the image doesn’t require one. After loading the image to $loadaddr and the FDT to $fdt_addr_r, boot with:

=> bootm $loadaddr - $fdt_addr_r
## Booting kernel from Legacy Image at 80060000 ...
   Image Name:   linux
   Image Type:   RISC-V Linux Kernel Image (uncompressed)
   Data Size:    1827316 Bytes = 1.7 MiB
   Load Address: 80000000
   Entry Point:  80000000
   Verifying Checksum ... OK
## Flattened Device Tree blob at 80400000
   Booting using the fdt blob at 0x80400000
   Loading Kernel Image
   Loading Device Tree to 00000000803f9000, end 00000000803ff473 ... OK

Starting kernel ...

The FDT is verified after the kernel is relocated, so it must be loaded high enough so that it won’t be overwritten. The default values for $loadaddr and $fdt_addr_r should provide ample headroom for most use-cases.

Flashing Images

SPI Flash

To flash data to SPI flash, first load it using one of the methods in Loading Images. Addiotionally, create some partitions as described in Partitions. Then use the mtd command:

=> sf probe
SF: Detected w25q128fw with page size 256 Bytes, erase size 4 KiB, total 16 MiB
=> mtd write linux $loadaddr 0 $filesize
Writing 2478162 byte(s) at offset 0x00000000

Note that in order to write a bootable image, a header and tailer must be added.


MMC writes are unsupported for now.

SPI Flash

Sipeed MAIX boards typically provide around 16 MiB of SPI NOR flash. U-Boot is stored in the first 1 MiB or so of this flash. U-Boot’s environment is stored at the end of flash.


There is no set data layout. The default partition layout only allocates partitions for U-Boot and its default environment

=> mtd list
List of MTD devices:
* nor0
  - type: NOR flash
  - block size: 0x1000 bytes
  - min I/O: 0x1 bytes
  - 0x000000000000-0x000001000000 : "nor0"
      - 0x000000000000-0x000000100000 : "u-boot"
      - 0x000000fff000-0x000001000000 : "env"

As an example, to allocate 2MiB for Linux and (almost) 13 MiB for other data, set the mtdparts like:

=> env set mtdparts nor0:1M(u-boot),2M(linux),0xcff000(data),0x1000@0xfff000(env)
=> mtd list
List of MTD devices:
* nor0
  - type: NOR flash
  - block size: 0x1000 bytes
  - min I/O: 0x1 bytes
  - 0x000000000000-0x000001000000 : "nor0"
      - 0x000000000000-0x000000100000 : "u-boot"
      - 0x000000100000-0x000000300000 : "linux"
      - 0x000000300000-0x000000fff000 : "data"
      - 0x000000fff000-0x000001000000 : "env"

To make these changes permanent, save the environment:

=> env save
Saving Environment to SPIFlash... Erasing SPI flash...Writing to SPI flash...done

U-Boot will always load the environment from the last 4 KiB of flash.

Pin Assignment

The K210 contains a Fully Programmable I/O Array (FPIOA), which can remap any of its 256 input functions to any any of 48 output pins. The following table has the default pin assignments for the BitM.

Pin Function Comment
IO_6   Not set
IO_7   Not set
IO_8 GPIO_0  
IO_9 GPIO_1  
IO_10 GPIO_2  
IO_11 GPIO_3  
IO_12 GPIO_4 Green LED
IO_13 GPIO_5 Red LED
IO_14 GPIO_6 Blue LED
IO_15 GPIO_7  
IO_17 GPIOHS_1  
IO_21 GPIOHS_5  
IO_22 GPIOHS_6  
IO_23 GPIOHS_7  
IO_24 GPIOHS_8  
IO_25 GPIOHS_9  
IO_30 GPIOHS_14  
IO_31 GPIOHS_15  
IO_32 GPIOHS_16  
IO_33 GPIOHS_17  
IO_34 GPIOHS_18  
IO_35 GPIOHS_19  
IO_36 GPIOHS_20 Panel CS
IO_37 GPIOHS_21 Panel RST
IO_38 GPIOHS_22 Panel DC
IO_39 SPI0_SCK Panel WR

Over- and Under-clocking

To change the clock speed of the K210, you will need to enable CONFIG_CLK_K210_SET_RATE and edit the board’s device tree. To do this, add a section to arch/riscv/arch/riscv/dts/k210-maix-bit.dts like the following:

&sysclk {
    assigned-clocks = <&sysclk K210_CLK_PLL0>;
    assigned-clock-rates = <800000000>;

There are three PLLs on the K210: PLL0 is the parent of most of the components, including the CPU and RAM. PLL1 is the parent of the neural network coprocessor. PLL2 is the parent of the sound processing devices. Note that child clocks of PLL0 and PLL2 run at half the speed of the PLLs. For example, if PLL0 is running at 800 MHz, then the CPU will run at 400 MHz. This is the example given above. The CPU can be overclocked to around 600 MHz, and underclocked to 26 MHz.

It is possible to set PLL2’s parent to PLL0. The plls are more accurate when converting between similar frequencies. This makes it easier to get an accurate frequency for I2S. As an example, consider sampling an I2S device at 44.1 kHz. On this device, the I2S serial clock runs at 64 times the sample rate. Therefore, we would like to run PLL2 at an even multiple of 2.8224 MHz. If PLL2’s parent is IN0, we could use a frequency of 390 MHz (the same as the CPU’s default speed). Dividing by 138 yields a serial clock of about 2.8261 MHz. This results in a sample rate of 44.158 kHz—around 50 Hz or .1% too fast. If, instead, we set PLL2’s parent to PLL1 running at 390 MHz, and request a rate of 2.8224 * 136 = 383.8464 MHz, the achieved rate is 383.90625 MHz. Dividing by 136 yields a serial clock of about 2.8228 MHz. This results in a sample rate of 44.107 kHz—just 7 Hz or .02% too fast. This configuration is shown in the following example:

&sysclk {
    assigned-clocks = <&sysclk K210_CLK_PLL1>, <&sysclk K210_CLK_PLL2>;
    assigned-clock-parents = <0>, <&sysclk K210_CLK_PLL1>;
    assigned-clock-rates = <390000000>, <383846400>;

There are a couple of quirks to the PLLs. First, there are more frequency ratios just above and below 1.0, but there is a small gap around 1.0. To be explicit, if the input frequency is 100 MHz, it would be impossible to have an output of 99 or 101 MHz. In addition, there is a maximum frequency for the internal VCO, so higher input/output frequencies will be less accurate than lower ones.

Technical Details

Boot Sequence

  1. RESET pin is deasserted. The pin is connected to the RESET button. It can also be set to low via either the DTR or the RTS line of the serial interface (depending on the board).
  2. Both harts begin executing at 0x00001000.
  3. Both harts jump to firmware at 0x88000000.
  4. One hart is chosen as a boot hart.
  5. Firmware reads the value of pin IO_16 (ISP). This pin is connected to the BOOT button. The pin can equally be set to low via either the DTR or RTS line of the serial interface (depending on the board).
    • If the pin is low, enter ISP mode. This mode allows loading data to ram, writing it to flash, and booting from specific addresses.
    • If the pin is high, continue boot.
  6. Firmware reads the next stage from flash (SPI3) to address 0x80000000.
    • If byte 0 is 1, the next stage is decrypted using the built-in AES accelerator and the one-time programmable, 128-bit AES key.
    • Bytes 1 to 4 hold the length of the next stage.
    • The SHA-256 sum of the next stage is automatically calculated, and verified against the 32 bytes following the next stage.
  7. The boot hart sends an IPI to the other hart telling it to jump to the next stage.
  8. The boot hart jumps to 0x80000000.

Debug UART

The Debug UART is provided with the following settings:


Resetting the board

The MAIX boards can be reset using the DTR and RTS lines of the serial console. How the lines are used depends on the specific board. See the code of for details.

This is the reset sequence for the MAXDUINO and MAIX BiT with Mic:

def reset(self):

and this for the MAIX Bit:

def reset(self):

Memory Map

Address Size Description
0x00000000 0x1000 debug
0x00001000 0x1000 rom
0x02000000 0xC000 clint
0x0C000000 0x4000000 plic
0x38000000 0x1000 uarths
0x38001000 0x1000 gpiohs
0x40000000 0x400000 sram0 (non-cached)
0x40400000 0x200000 sram1 (non-cached)
0x40600000 0x200000 airam (non-cached)
0x40800000 0xC00000 kpu
0x42000000 0x400000 fft
0x50000000 0x1000 dmac
0x50200000 0x200000 apb0
0x50200000 0x80 gpio
0x50210000 0x100 uart0
0x50220000 0x100 uart1
0x50230000 0x100 uart2
0x50240000 0x100 spi slave
0x50250000 0x200 i2s0
0x50250200 0x200 apu
0x50260000 0x200 i2s1
0x50270000 0x200 i2s2
0x50280000 0x100 i2c0
0x50290000 0x100 i2c1
0x502A0000 0x100 i2c2
0x502B0000 0x100 fpioa
0x502C0000 0x100 sha256
0x502D0000 0x100 timer0
0x502E0000 0x100 timer1
0x502F0000 0x100 timer2
0x50400000 0x200000 apb1
0x50400000 0x100 wdt0
0x50410000 0x100 wdt1
0x50420000 0x100 otp control
0x50430000 0x100 dvp
0x50440000 0x100 sysctl
0x50450000 0x100 aes
0x50460000 0x100 rtc
0x52000000 0x4000000 apb2
0x52000000 0x100 spi0
0x53000000 0x100 spi1
0x54000000 0x200 spi3
0x80000000 0x400000 sram0 (cached)
0x80400000 0x200000 sram1 (cached)
0x80600000 0x200000 airam (cached)
0x88000000 0x20000 otp
0x88000000 0xC200 firmware
0x8801C000 0x1000 riscv priv spec 1.9 config
0x8801D000 0x2000 flattened device tree (contains only addresses and interrupts)
0x8801F000 0x1000 credits