Setting Up

UART Serial Console

The Megrez board includes a reserved UART debugging serial port, which can be used to view system boot logs or log in to the console after the system has booted to execute terminal commands.

USB to Serial Cable

The pin definitions of a common USB to TTL serial cable are as follows:

Connecting the Serial Port

The debugging serial port operates at a 3.3V level. As shown in the diagram below, connect the USB to TTL serial module, but do not connect the red wire.

Milk-V Megrez	<--->	USB to TTL Cable
GND	<--->	Black wire
RX	<--->	Green wire (TX)
TX	<--->	White wire (RX)

The default serial port parameters for Megrez are as follows:

baudrate: 115200
data bit: 8
stop bit: 1
parity  : none
flow control: none

Release the memory used by the NPU

After the system starts, the system memory size should be smaller than the nominal value. For example, for a version with 16G memory, the memory is about 10G through the command:

$ free -h
               total        used        free      shared  buff/cache   available
Mem:           9.7Gi       471Mi       8.9Gi       9.3Mi       426Mi       9.2Gi
Swap:             0B          0B          0B

This is because part of the memory is allocated to the NPU. If you do not need the NPU function, you can use the following method to release the memory occupied by the NPU.

осторожно

Releasing the memory used by the NPU will cause the loss of the NPU and related codec functions! If the system memory meets the usage requirements, it is not recommended to release the memory used by the NPU!

Replace the device tree

Download the dtb file (download the corresponding dtb file according to the system image version you use):

eic7700-milkv-megrez-no-npu.dtb

Back up the dtb file used by the original system:

sudo cp /boot/dtbs/linux-image-6.6.66-win2030/eswin/eic7700-milkv-megrez.dtb /boot/dtbs/linux-image-6.6.66-win2030/eswin/eic7700-milkv-megrez.dtb_bak

Replace the downloaded dtb file:

sudo cp -rf eic7700-milkv-megrez-no-npu.dtb /boot/dtbs/linux-image-6.6.66-win2030/eswin/eic7700-milkv-megrez.dtb

Restart the device after syncing:

sync
sudo reboot

After restarting, check the memory information again, it has become about 16G:

$ free -h
               total        used        free      shared  buff/cache   available
Mem:            15Gi       858Mi        14Gi        10Mi       764Mi        14Gi
Swap:             0B          0B          0B

A one-time temporary method

If you only need to temporarily test and release the memory occupied by NPU, you can do so by modifying the configuration in u-boot. This method requires the use of a serial line to enter the UART serial console for operation.

After the system is powered on, pay attention to the log information output in the PC serial terminal. After the prompt Autoboot in 5 seconds appears, press any key to interrupt the u-boot startup and enter the u-boot command line. (If you have updated u-boot, you need to press the s key to enter the u-boot command line)

Execute the following command:

=> fdt mmz mmz_nid_0_part_0 0x300000000 0x1000

Then enter the boot command to start:

=> boot

After entering the system, check the memory information and it has become about 16G:

debian@rockos-eswin:~$ free -h
               total        used        free      shared  buff/cache   available
Mem:            15Gi       472Mi        15Gi       9.3Mi       286Mi        15Gi
Swap:             0B          0B          0B

информация

This method is only valid for this startup. After powering on again, the NPU will restore the system's default memory allocation.